BUG: #12624 where Panel.fillna() ignores inplace=True
- [x] closes #12624 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Fixes #12624

Author: Pedro M Duarte pmd323@gmail.com

xref #12633 
 duped in #12633 

thanks!
 Thanks for the feedback, Jeff! I wanted to get home to fix it but seems like it has been taken care of.  Will look for another easy one I can try to help with. 
 @PedroMDuarte np. this actually happens pretty rarely when 2 people are working on the same issue. In any event lots of issues to try for!
