BUG: additional keys in groupby indices when NAs are present
- [x] closes #9304
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

`lib.indices_fast` included logic to skip over null values (-1's), but did not include the case where the first index is null. Currently this function is only used in `sorting.get_indexer_dict` where the case of all null group_index is handled separately.