WIP: ENH implement fill_value for _combine_series_frame #13488
- [ ] closes #13488
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @goblinqueen! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/ops.py`](https://github.com/goblinqueen/pandas/blob/b88df660c2b0d305d891fb82434d75e8bdeadeb3/pandas/core/ops.py) !

 Take a look at ops.fill_binop, that is probably closer to the needed implementation.

Also will definitely need tests. closing as stale. if you'd like to continue, pls ping.