Fixed regression of Multi index with NaN
- [x] closes #22247
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
As pull #19074 changed compute of hashtable from `self.values` to `self.codes` and `self.levels`, NaN values doesn't cover.
I just simply fixed this regression the special case, by check return value from the hashtable.