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 believesbelieve 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 :)