TST: Multiindex slicing with NaNs, unexpected results for #25154
- [x] closes #25154
- [x] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Added test case for Multiindex slicing with NaNs. 
Executed linting test based on the instructions provided here and 23 errors were reported, however none regarding the python test file modified. 
