Bug fix GH 29624: calling str.isalpha on empty series returns object …
…dtype, not bool

Added dtype=bool argument to make _noarg_wrapper() return a bool Series

- [x] closes #29624  
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

thanks @ellequelle very nice!