FIX: series.fillna doesn't shallow copy if len(series) == 0
- [x] closes #32543
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

/azp run
 <samp>
Commenter does not have sufficient privileges for PR 32733 in repo pandas-dev/pandas<br>

</samp> @jreback Adapted the isnew section and CI is green @jreback btw, is there any chance I could get privileges to run `/azp run`? @SaturnFromTitan added you to pandas-triage

you can now label & close issues (i think u can prob trigger CI not sure about that) sweet, thanks a lot @jreback! Great thanks @SaturnFromTitan and congrats on the triage role - well deserved! Thanks for the kind words @WillAyd :)