BUG: fix Series.where(cond) when cond is empty
- [ ] closes #34592
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

xref #21947 for fix for DataFrame.where

thanks @simonjayhawkins 