TST: Add a test for fillna in PeriodArray
- [x] closes #24537 

@phofl @noatamir ready for review @phofl should I still do something before this PR can be merged? Right now it's failing on pre-commit, but it's not related to my changes. Sorry, was a bit busy yesterday.

pre-commit is failing, otherwise this lgtm  Looks like pre-commit failure came from something else, merged main to check thx @mstazherova  Cool, thank you! Excited to do more :)