validateID(id) {
        if (!id) {
            return undefined; // to indicate ID does not exist
        }
        if (!Array.isArray(id) || id.length !== 1
        || typeof id[0] !== 'string') {
            return errors.MalformedXML;
        }
        if (id[0] === '') {
            return undefined;
        }
        return true;
    }
_validator.validateID - check value of optional ID
@param {string[]} id - array containing id string
@return {(Error|true|undefined)} - Arsenal error on failure, true on
  success, undefined if ID does not exist