Fix regression for is_monotonic_increasing with nan in MultiIndex
- [x] closes #37220
- [x] closes #27498 (closes that one too probably, because we need this case)
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Found this while debugging a different issue. 