TST: Test Series' settitem with Interval and NaN
Fixes #27937. As described in that issue, assigning a mixture of NaN and
Interval values into a slice of a categorical-typed series would raise
an exception.  This is no longer the case.

- [x] closes #27937
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

thanks @ElDeveloper very nice! Thanks so much for your feedback!