Nagarjuna (ca. 150–250 CE) doesn’t appear to give emptiness an ontological cover at least in the context he uses the term here. He says:
23. The ambrosial teaching of emptiness (shunyata) aims at abolishing all conceptions (samkalpa). But if someone believes in that emptiness you [have declared] he is lost” (Lokâtîtastava). (trans., Chr. Lindtner.)
To believe in emptiness beyond empty things themselves, which are illusory in every way, is to believe in that emptiness giving it ontological cover.
Our mental conceptions and contructs, including imagination and abstraction, fall under samkalpa which emptiness absolishes the value of. Every samkalpa is also a dependent origination just like a pot made of clay or rope made from munja grass. The originated thus lacks inherent nature (nishsvabhava) and ultimacy. It is nothing desirable.
Samkalpa have no reality—there is no exception. But we should not conclude that samkalpa spring from nothing or from other samkalpa. That which is the fundamental substance of samkalpa by which it is composed, is what the sage cognizes—not any samkalpa. This substance or tathatâ is not a dependent origination, either, like a sandcastle made of sand or a gold ring made from gold or a mental image of the Buddha which is a construct of pure Mind or tathatâ. In the Mahaprajnaparamita Shastra, attributed to Nagarjuna he says:
“Tathatâ is the truly real nature of all things. For example, whether it is in a palace or in a humble hut, whether it is the sandalwood or just the ordinary wood that is being burnt, in regard to the space (âkâsa) in both these places there is no difference. Of all things, when one seeks to know the (ultimately) true nature, (one finds that) all that is just the tathatâ (the undivided, non-dual dharma). (And where all is one and devoid of distinction how can there be any speech?) For that reason, the Buddha delighted at heart in keeping silent, when He first realized the bodhi He did not like to teach the dharma. He knows that it is difficult for ordinary minds to comprehend the profound dharma. (563c)” (Venkata Ramanan, Nagarjuna’s Philosophy, pp. 273-274).