BUG: IntervalIndex is_monotonic, get_loc, get_indexer_for, contains with np.nan
- [x] closes #41831
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

actually can you rebase (as merged #41870 should be fine) rebased, whatsnew added, and greenish