BLD: ABCSeries mapper maps unmapped categories to "" instead of NaN
- [x] closes #29162
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


I think this is going to break a lot of things and maybe not the direction we want. Did you see @jorisvandenbossche comment in the original post? Yeah, just saw it. I'll close this pr and think about another approach to resolve the issue