Fix test_groupby_unaligned_index for pandas 2.0

[j-bennet]

Fixes an upstream failure with Pandas 2.0:

dask/dataframe/tests/test_groupby.py::test_groupby_unaligned_index[disk]: AssertionError: assert FrozenList(['a', None]) == FrozenList([None])
  At index 0 diff: 'a' != None
  Left contains one more item: None
  Full diff:
  - FrozenList([None])
  + FrozenList(['a', None])
  ?             +++++
dask/dataframe/tests/test_groupby.py::test_groupby_unaligned_index[tasks]: AssertionError: assert FrozenList(['a', None]) == FrozenList([None])
  At index 0 diff: 'a' != None
  Left contains one more item: None
  Full diff:
  - FrozenList([None])
  + FrozenList(['a', None])
  ?             +++++

• Xref Fixes related to group_keys default change in pandas 2.0 #9855.
• Xref ⚠️ Upstream CI failed ⚠️ #9736.
• Xref API: User-control of result keys in GroupBy.apply pandas-dev/pandas#34998.

• Tests added / passed
• Passes pre-commit run --all-files

[jrbourbeau]

Thanks @j-bennet!