For Nāgārjuna, as for the Buddha in the early texts, it isnot merely sentient beings that are "selfless" or non-substantial; all phenomena are without any svabhāva, literally "own-being" or "self-nature", and thus without any underlying essence. They are empty of being independently existent; thus the heterodox theories of svabhāva circulating at the time were refuted on the basis of the doctrines of early Buddhism. This is so because all things arise always dependently: not by their own power, but by depending on conditions leading to their coming into existence,as opposed to being.
So, empty means empty of inherent existence. Empty of substance.
Since this is obvious, everything is accident, motion, impermanence.The dharmas are empty like everything else.
Once this is understood, the heart of Buddhist Dharma is clear.