How to cite that something is *not* on Wikipedia
I don't think one needs a citation for the fact that no citation exists. If you're identifying a gap in the regular literature, you can say something like 'to the authors' knowledge, no study of this kind has been published' or words to that effect and you don't need a citation to confirm that you are identifying a gap in the literature.

I find it highly unlikely that you would be criticised for wording along the lines of 'at the time of writing (possibly give date), no article exists on this topic at Wikipedia' and then maybe give a general citation for Wikipedia as a whole. As Wikipedia time stamps every edit to every article, this is something that could very easily be verified at any time in the future. I'm not sure having a Wikipedia page or not is the best measure of obscurity, but that's a different question entirely.