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. 
@phofl can you resolve conflict. @simonjayhawkins Done this looks fine. can you run some asv's to see if any perf diffs here? (prob don't need to add, want to see the existing ones). @jreback 
```
 before           after         ratio
     [57c06d9b]       [31ef0b98]
     <36915^2>        <37220>   
-        63.5±5ms       56.5±0.4ms     0.89  multiindex_object.SetOperations.time_operation('non_monotonic', 'string', 'symmetric_difference')

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.
```
Non monotonic indices increased (not surprising). No changes otherwise thanks @phofl  @meeseeksdev backport 1.1.x