(wip)BUG: make sure combine doesn't alter dtype for nullable dtypes
- [ ] closes #31899 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Haven't added new tests, but have changed the expected return dtype in existing ones

EDIT
----
This is the wrong fix - for, say
```
s1.combine(s2, lambda x, y: x<y)
```
the dtype _would_ be expected to change (in this case to bool)