Our index contained empty groups (e.g., from a resampling). If we
        reduced on that dimension, we want to restore the full index.

def _maybe_restore_empty_groups(self, combined):
        """Our index contained empty groups (e.g., from a resampling). If we
        reduced on that dimension, we want to restore the full index.
        """
        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