def mk_optional_string(data, jfield):
    """
    Returns either the string represented in a json structure, or an empty string if it's not present
    :param data:   json that may contain the jfield
    :param jfield: the name of the field
    :return if the field exists then it's returned; otherwise an empty string is returned
    """
    val = data.get(jfield)
    if val is None:
        return ""
    else:
        return val