DEPR: Add FutureWarning when matching NaN with NaN in merge/join
- [x] xref #32306 (and a few others, will add them later
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

cc @WillAyd 

I started with raising a FutureWarning when merging nan on nan. This works, if ``left_index`` and ``right_index`` are not both True.

If both are True, we are dispatching to the Index methods. Should we deprecate this for indexes too? Additionally, should we remove this for ``intersection`` of index in the future too? Will continue after receiving feedback :)
This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. not averse to doing this @phofl if you can add a test that shows the warning. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @phofl if you can update or close. Closing this for now, have to evaluate here. To be consistent this would influence a lot of index methods too, if nan should no longer match nan