Make sure a "Reference" database ID is empty.

    :type database_id: unicode
    :param database_id: The ``database_id`` field from a "Reference" protobuf.

    :raises: :exc:`ValueError` if the ``database_id`` is not empty.

def _check_database_id(database_id):
    """Make sure a "Reference" database ID is empty.

    :type database_id: unicode
    :param database_id: The ``database_id`` field from a "Reference" protobuf.

    :raises: :exc:`ValueError` if the ``database_id`` is not empty.
    """
    if database_id != u"":
        msg = _DATABASE_ID_TEMPLATE.format(database_id)
        raise ValueError(msg)