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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)