code: function (input) {
      debug('optionalTextFilter(%s)', input);
      if (_.isString(input)) return input;
      if (_.isBoolean(input)) return (input ? '' : undefined);
      if (_.isNumber(input)) return '' + input;
      return undefined;
    }
docs: Given some text or the empty string return it.
  If we receive true return empty string.
  If we receive a number convert it to a string and return it.
  Otherwise return undefined.
  
  @param {(string|boolean|number|undefined|null)} input
  @returns {(string|undefined)}