BUG: Fix divmod fill value, closes #26987
- [x] closes #26987
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Takes over from #27130 by implementing dispatch_fill_zeros.  This is unfortunately similar to dispatch_missing, and eventually the two functions will be merged.  ATM I'm trying to do exactly that in the branch for 27130, having lots of trouble pop up with sparse and IntegerArray.
this supersedes #27130 right? yes, but I'm keeping that open because that branch has evolved into addressing other bugs thanks @jbrockmendel 