BUG: SeriesGroupBy.value_counts() raising error on an empty Series
- [x] closes #39172
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

@jreback Green and ready to go!  looks good. can you merge master and ping on green (the reason is just merged a PR that changes the hashing of .value_counts()); it should be fine, just making sure. @jreback I've merged the master, and it's green thanks @AnnaDaglis keep em coming!