code: def _maybe_restore_empty_groups(self, combined):
          
          if (self._full_index is not None and
                  self._group.name in combined.dims):
              indexers = {self._group.name: self._full_index}
              combined = combined.reindex(**indexers)
          return combined
docs: Our index contained empty groups (e.g., from a resampling). If we
          reduced on that dimension, we want to restore the full index.