BUG-25061 fix printing indices with NaNs
- [X] closes #25061- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

The `FloatArrayFormatter` leaves a space in front of the floats it formats (expected behaviour as demonstrated by [this test](https://github.com/pandas-dev/pandas/blob/638ddebaa7da1e569836a5b89593b120dbbf491c/pandas/tests/io/formats/test_format.py#L2395)). To avoid misaligned values when printing indices, `NaN` values must be treated similarly.