Why should Mahayana practitioners strive for anything at all? In the Theravada tradition:


There is no self in all phenomena (including the five aggregates).
The five aggregates and the rest of nature and the world is always changing and not permanent (anicca).
Emptiness is only about the emptiness of self in all phenomena (including the five aggregates). The aggregates and the world definitely exist. These are not empty.
Suffering definitely exists (first noble truth).
The Theravada practitioner has to strive to become free from suffering. Nibbana exists, and is permanent and unchanging.


Whereas, in the Mahayana tradition:


There is no self in all phenomena (including the five aggregates).
The five aggregates and the rest of nature and the world is always changing and not permanent (anicca).
Emptiness refers to the emptiness of everything (including Nirvana) - the self, the five aggregates, the world, nature etc.
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."
According to the Heart Sutra, "There is No Wisdom, and There is No Attainment Whatsoever."


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?
Well in Mahayana, just like in Theravada, we strive for the end of striving. As @ChrisW said, Heart Sutra is written from the perspective of completion, the end of striving. This is called "taking the result as the path". Meaning, instead of discovering the result step by step, you try the result  on, like a hat, until you internalize it through getting used to it. 

Another point of Heart Sutra is to serve as an antidote for attachment and reification of the raft of Dharma. Mahayana's issue with Theravada is that they sometimes get so obsessed with Dharma concepts to the point that Dharma becomes a problem, an obstacle in itself. So by reminding of its inherent emptiness we are hoping to help the student come back to the natural state, so they can experience the groundlessness and positionlessness in which there is no more need for striving.

Now, to just abandon all effort without having mastered the peaceful mind would be fooling oneself. Which is why in real life most Mahayana practitioners have to work on essentially the same practices as Theravada, whether before or after realization of emptiness.

Wouldn't it be nice if one could attain dispassion and unbinding upon hearing that everything is contrived and absolutely pointless? But habitual patterns and karmic inertia are difficult to overcome... 

Although certainly in the absolute sense the universe does not care if you're enlightened or still suffer. It remains as lacking any solidity or certainty to rely on as it always were.