Javascript error when telephone element does not have a placeholder.

Problem/Motivation

When using a telephone element on a webform without setting the "placeholder" attribute, a javascript error is thrown upon validation: "Uncaught TypeError: str is undefined"

Steps to reproduce

Add a telephone element to a webform, do not specify a placeholder. Fill out the form with an incorrect phone number, the validation error message triggers the js error.

Proposed resolution

In webform.element.telephone.js:

if (!$telephone.intlTelInput('isValidNumber')) {
  $telephone.addClass('error');
  var message = Drupal.t('The phone number is not valid. (e.g. @example)', {'@example': $telephone.attr('placeholder')});
  $error.html(message).show();
  return false;
}

Ensure that the placeholder attribute exists/has a value, otherwise do not use it as argument for the message.

Remaining tasks

Write patch, review, test, commit.

User interface changes

The validation error message might change a bit if there is no placeholder attribute

API changes

None.

Data model changes

None.