[BUG] don't mangle null-objects in value_counts
- [x] closes #42688
- [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

Null-like values are no longer mangled in `value_counts`.

This was overlooked in #22296, as decision was made not to mangle null-like values (np.nan, None, pd.NaT and so on).

It also has impact on `mode`, as it uses `value_counts` under the hood.