BUG: Fix sorting by column named None in DataFrame.sort_values (GH#61512)
Fixes a bug in `DataFrame.sort_values` where sorting by a column explicitly named None raised a `KeyError`. Added a conditional check to correctly retrieve and sort the column when `None` is used as a label.

- [x] closes #61512
- [x] Tests added and passed
- [x] All code checks passed via pre-commit
- [x] Added an entry to the latest whatsnew file
Thanks @iabhi4 