BUG: raise when sort_index with ascending=None
- [ ] closes #39434
- [ ] 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 validation of ``ascending`` for ``sort_index`` methods.
Proposed a solution to raise ``ValueError`` when either of the elements of ``ascending`` turned out to be ``None``.