Is nirvana a conceptual construction?

Is nirvana a conceptual construction - empty in that way? For any / only some Buddhists. I'm just trying to figure out how extinction can avoid the extremes of eternalism and annihilation. If it is a conceptual construction in the sense of having no reality outside what it is in conventional designation (blissful etc.), that *might* be an answer. If some rock has no qualities that we cannot define, then maybe there is nothing to add to its existence, nothing to puzzle over and ask why or how.