BUG: Index.get_loc(np.nan) non-unique non-monotonic
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

The tests file will need to be rebased following #43705.
looks fine, user facing? > looks fine, user facing?

yah, will add whatsnew Whatsnew added + green looks good. can you rebase again and ping on green. ping