REGR: rename_axis with None should remove axis name
- [X] closes #25034
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Fix amounts to using [~`com.sentinel_factory()`~](https://github.com/pandas-dev/pandas/blob/ea013a254847fbc5551e3ec91a468370c7118589/pandas/core/common.py#L386-L390)  `object()` as a sentinel value instead of `None`.

Had to add a `sentinel` kwarg to `_construct_axes_from_arguments` since it's responsible for the `index` and `columns` kwargs, which need to be treated similarly to `mapper`.

One change in behavior that I kept from 0.24.0: passing nothing to `rename_axis` does nothing, i.e. `s.rename_axis()` does not alter the existing axis name(s).  In 0.23.4 this would raise a `TypeError`, as `mapper` was a required positional argument.
