BUG: pd.MultiIndex.get_loc(np.nan)
MultiIndex.get_loc can find nan when input is NA value(e.g. nan, None)

- [ ] closes #19132
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @proost! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-10-10 03:33:36 UTC @toobaz care to take a look?