def missing_to_default(field, default):
    """
    Function to convert missing values into default values.

    :param field: the original, missing, value.
    :param default: the new, default, value.
    :return: field; the new value if field is an empty string, the old value
             otherwise.
    :rtype: any
    """

    if field == '':
        return default
    else:
        return field