Nāgārjuna
All things are empty of inherent existence.
Misconceptions
Nagarjuna teaches that nothing exists (it is nihilism).
This is the charge his opponents already make inside his own book (24.1 to 24.6), and he rejects it. For him "empty" means "without own-nature", not "non-existent": emptiness is the name of dependent origination (24.18). He refuses both "it exists" and "it does not exist": the first leads to believing in the eternal, the second to annihilation (15.10), and for whom emptiness is tenable, everything is tenable (24.14).
Emptiness is an ultimate reality hidden behind the world.
That is not the text's reading. Emptiness is not one more thing: it is dependent origination itself, and a mere "designation" (24.18). Nagarjuna warns that those for whom it becomes a view to hold are incurable (13.8). His commentator Candrakīrti says it plainly: emptiness, dependent designation and middle way are names of dependent origination alone. (Candrakīrti is not Nagarjuna.)