Why should Mahayana practitioners strive for anything at all?

Theravadin Buddhism does not refer to sunyata (emptiness). It is best viewed as a psychology of karma and method of overcoming misleading karma (suffering). Mahayana Buddhism strives to overcome self-centeredness. The experience of sunyata is considered to be the only way to thoroughly erase this self-centeredness. Hence the Mahayana practitioner does indeed have a goal. The fact that you raise the question you have asked indicates a very superficial understanding of Mahayana Buddhism. Understanding karma would be a far more profitable pursuit of insight.

There's different answers, and different people even will tell you different things, let alone different texts.

I have two answers. The 1st is that, because of meditation and good teachers, sentient beings can have faith in their Buddha Nature, and work to fully realise that, like anyone can work at their ideals, without fully understanding them.

Also maybe parinirvana (the complete extinction of the skandhas) may have some value.

The way I understood emptiness was through the framework elaborated by the Dalai Lama in his books. Within this framework, emptiness doesn't mean 'nothingness', but rather emptiness of intrinsic nature. This means, by the philosophy of Nagarjuna, that things are dependently originated. They arise as conditioned by cause and effect.

Also, the framework of Nagarjuna distinguishes between conventional and ultimate reality. Conventional means how things appear in the everyday, while ultimate truth is perceiving reality from a perspective of wisdom. So, for example, there are conventional realities, a chair does exist, and this is within the view of dependent origination linked with causality. However, there is an ultimate reality whereby things exist as emptiness, that is lacking intrinsic essence.

I would liken this to a chair existing as a chair (conventional) but also as interacting forces at different scopes, i.e. atoms, molecules, perceptions, etc. all interacting together in a non-definable way (ultimate).

So, to answer your question, there are people suffering in the world, but these individuals lack intrinsic essence: ultimately, when analyzed with discriminating wisdom, these individuals fail to intrinsically exist. In simple terms, there are people needing help but they exist in a way reliant on emptiness.

The Heart Sutra, once again from the Dalai Lama's books, would be definitive rather than requiring interpretation. In this sense, it explains reality from the point of view of emptiness, from ultimate reality. So, indeed, from ultimate reality nothing exists, and the Heart Sutra is elaborating upon that reality, is showing this reality. We don't need to interpret within it: ultimate reality is portrayed in it. However, the conventional reality of every day appears in stark contrast with its tenets. (But in a sense, not really. Reality is ultimate, just as atoms are 'truer' than the chair: it is our perception that shows us the conventional.)

So ANSWER PART 1: There is a conventional reality underlying the ultimate nature of phenomena. There are real dhammas like striving in this reality.

Then, I would suggest the Mahayana viewpoint emphasizes compassion. I would liken compassion and emptiness' interaction as a bunch of moths flying around a lamp. The shadows of these moths are like the suffering of beings, tied with the moths. And, sometimes these shadows reflect the moths in form (conventional) but sometimes the shadows don't even resemble the moths (ultimate). However, these are the SAME shadows, only in different moment, different aspects. Compassion is like the lamp: it always interacts with the suffering, the shadows, but not necessarily in an obvious, categorically separate way.

So, no matter if the shadows are distinct, fused into a mass, light, dark, discernible or vague: in all cases, compassion brightens and relieves suffering as the lamp dispels the shadows. What changes is whether we are categorizing the shadows as intrinsic entities, distinct dhammas (e.g. moths), or merely seeing them as they actually are: a bunch of shadows.

Hence, after this perhaps too elaborate simile, ANSWER PART 2 is that compassion targets suffering even without individual entities to strive for. It connects with suffering causing positive karma, future positive rebirth, and striving altogether.

Hopes this helps,

Eggman.

PS: The books I refer to are here, here, here, and here.

Emptiness refers to the emptiness of everything (including Nirvana) - the self, the five aggregates, the world, nature etc.

Indeed it does. Everything is empty of inherent existence. Nothing exists inherently or essentially. Not the Buddha, not Nirvana, not the 4NT, not suffering, not the cessation of suffering... Why? Because inherent existence is an impossible mode of existence.

Still, all of these definitely exist: the Buddha, Nirvana, the 4NT, suffering, the cessation of suffering, and the path leading to the cessation of suffering.

According to the Heart Sutra, "There is No Truth of Suffering, Of the Cause of Suffering, Of the Cessation of Suffering, Nor of the Path."

You are missing an important caveat that is stressed again and again in all the commentary literature on the Heart Sutra. There is no inherent or essential Truth of Suffering, no inherent or essential Cause of Suffering, no inherent or essential Cessation of Suffering, and no essential or inherent Path leading to the Cessation of Suffering.

There... fixed to avoid misunderstanding.

So, why should the Mahayana practitioner strive for anything at all, if everything is emptiness (nothing really exists), and there is no suffering at all, and there is no wisdom to be gained, and there is nothing at all to attain?

Because illusion-like suffering, wisdom, and attainments do exist. Again, if you insist upon falling to the extreme of nihilism or insist that this is what proponents of the Middle Way are doing, then you are indicting the Buddha himself as you yourself know he propounded the Middle Way free from the extremes.

Consider: essentialists who believe in atman can and do regularly use entirely analogous misunderstandings and logics to indict the Theravada. That they are capable of these misunderstandings does not render them true and correct :)

Because no amount of truth is going to keep unrealized people from suffering from delusion. The real work is the liberation of all sentient beings.

I think the Heart Sutra may be analogous to the Brahmana Sutta (SN 51.15):

So it is with an arahant whose mental effluents are ended, who has reached fulfillment, done the task, laid down the burden, attained the true goal, totally destroyed the fetter of becoming, and who is released through right gnosis. Whatever desire he first had for the attainment of arahantship, on attaining arahantship that particular desire is allayed. Whatever persistence... Whatever intent... Whatever discrimination...

Before that happens:

In that case, brahman, let me question you on this matter. Answer as you see fit. What do you think: Didn't you first have desire, thinking, 'I'll go to the park,' and then when you reached the park, wasn't that particular desire allayed?"

"Yes, sir."

"Didn't you first have persistence, [etc.]

One way in which the Mahayana might be supposed to differ from the Theravada, perhaps, is that the Heart Sutra is ascribed to Avalokiteśvara ("who embodies the compassion of all Buddhas").

But the Theravada tradition too has an answer to the question "why would a Buddha, why did the Buddha, strive?" (e.g. for those who have little dust in their eyes; or Mahayana for all sentient beings).

Maybe some Mahayana practitioners don't strive: see for example this answer; see also this answer.

To give a more specific or less theoretical example, why did Thich Nhat Hanh strive?

Oprah: Thank you for the honor of talking to you. Just being in your presence, I feel less stressed than when the day started. You have such a peaceful aura. Are you always this content?

Nhat Hanh: This is my training, this is my practice. And I try to live every moment like that, to keep the peace in myself.

Oprah: Because you can't give it to others if you don't have it in yourself.

Nhat Hanh: Right.

Oprah: I see. I know that you were born in Vietnam in 1926. Is there any wonderful memory of your childhood that you can share?

Nhat Hanh: The day I saw a picture of the Buddha in a magazine.

Oprah: How old were you?

Nhat Hanh: I was 7, 8. He was sitting on the grass, very peaceful, smiling. I was impressed. Around me, people were not like that, so I had the desire to be like him.

Sure, why not? It's like one strives to not strive. It would be nice if we could just rip the coating of defilement off of ourselves to reveal the Buddha inside. It take's awhile to get naked, we got to strive but we don't have to strive to be a Buddha that is wearing a defilement suit.

                                         AND 

There is No Truth of Suffering, the Cause of Suffering, the Cessation of Suffering, Nor of the Path.... There is No Wisdom, and There is No Attainment Whatsoever

These kind of teachings are correct but can be misleading. The teachers mean that none of these things exist in "ultimate reality" but they do exist in "conceptual reality". We aren't trying to vilify conceptual reality. The idea is to experience ultimate reality and know the difference between The conceptual and ultimate realities. If we didn't have conceptual reality then we wouldn't be able to talk to each other , no step by step directions, there would be no BuddhismSE forum and there could be no suttas.

-Metta :)

I'll preface my answer by suggesting that you find a qualified teacher. That is to say, someone with years of formal study in a monastic setting leading to a qualification such as Geshe or Ajahn or equivalent.

Comparing Theravada and Mahayana is going to get you a lot of biased answers. The two traditions organize ideas in different ways and use terms differently, and this can lead to confusion. Pick one, and ignore the other until you get to the point where you have a really good handle on the teachings.

One of the problems we have in studying Buddhism (or any "foreign" philosophy for that matter) is that we use English words for concepts that don't really match those words precisely, and our understanding is tainted by the English meaning. It is often better to use the Pali or Sanskrit (or even Tibetan) terms instead, so as to avoid this taint.

"Emptiness" doesn't mean nothingness.

When we say "emptiness" we mean "devoid of inherent existence". That piece of hardened clay you call a "cup" and immediately ascribe properties to such as "used for drinking coffee" and "beautiful" isn't a cup from it's own side. You project those properties onto it. To that fly on your wall, it isn't a cup at all.

Similarly, you think of yourself as "me" rather than recognizing that "you" are a collection of physical components and mental processes none of which individually contain the an independent or eternal "you-ness".

So then, if we only really have as much "self" as a pile of leaves, why do we need to help each other? Because we all still suffer. Just because that suffering is also temporary and dependent on causes and is therefore also empty, doesn't mean it doesn't suck in the moment.

Metta