Helper:  convert field paths to message.

def _normalize_projection(projection):
        """Helper:  convert field paths to message."""
        if projection is not None:

            fields = list(projection.fields)

            if not fields:
                field_ref = query_pb2.StructuredQuery.FieldReference(
                    field_path="__name__"
                )
                return query_pb2.StructuredQuery.Projection(fields=[field_ref])

        return projection