 BUG: treat nan-objects the same way float64-nans are treated - all na…  
…ns are from the same equivalence class (GH22119)

- [x] closes #22119
- [x] closes #22148
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
