BUG: groupby with no non-empty groups, #21624
Should close #21624. Didn't see any issues running test_fast.sh on my laptop so I don't think it broke anything in the process. The problem here is that when there is only a null group, then `self.result_index` is empty, but `ids` is just a list of -1s, so a ValueError is raised when trying to create the series. 