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.