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

also pls add a whatsnew note to 1.3, bug fixes under categorical is fine.

so i guess this was just not tested before. @jreback

> also pls add a whatsnew note to 1.3, bug fixes under categorical is fine.

No worries.  I think it might be more consistent with past whatsnew files to place it under Groupby/resample/rolling though.

Happy to do what I'm told though 😄 
 thanks @venaturum  might resolve https://github.com/pandas-dev/pandas/issues/32471 or https://github.com/pandas-dev/pandas/issues/25970