BUG: Series.isnull() respect use_inf_as_na
- [x] closes #50495 
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

@phofl 

> Please always add tests

It seems that I misunderstood the checkbox (it's my first time contributing to this repo). I thought it meant that my code was passing tests which were already there but I have now realized that it is referring to any new tests that I have added (which I haven't since there was already a test for it). Is that making sense or am I still wrong? Yes correct, every fix should include a test that did not pass before  any update on this? Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.