BUG: GroupBy.idxmax/idxmin with EA dtypes
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@jorisvandenbossche thoughts on how/where to handle the skipna kwarg?