BUG: Series[sparse].where losing fill_value
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Excellent.  This along with #45673 and #45582 put us within striking instance of sharing some more Block methods