In early Buddhism, as I understand, ’empty’, as an adjective, was used much more frequently than ‘emptiness‘, as a noun, and usually was used as a negation. For example, “Insofar as it is empty of a self or of anything pertaining to a self: Thus it is said, Ānanda, that the world is empty” (SN 35.85). However, later Mahayana, Vajrayana, Zen, etc. authors began using emptiness as a noun and as an affirmation, equating it with a monistic Buddha-nature essence (Sanskrit: tathāgatagarbha). This is one of the most fundamental differences, and conflicts, I have seen between earlier and later Buddhism.