BUG: GH38672 SeriesGroupBy.value_counts for categorical
Unobserved categories in Series were being dropped in value_counts, which was inconsistent with Series.value_counts

- [x] closes #38672
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
