Clear tokens with no values

I noticed that when i use tokens as default values for form fields, the tokens are not cleared if no value was found. For example, my logged in users first name, last name etc is filled in as expected in the form fields, but when an anonymous user view the form the tokens are displayed as plain text, because no values were found.

It seems like you have to set a 'clear' flag in the options array for Token::replace to make this happen.

In YamlFormElementBase.php, in the prepare() function, this seems to solve the issue for me:

$token_options = [
  'clear' => TRUE,
];
foreach ($element as $element_property => $element_value) {
  // Most strings won't contain tokens so lets check and return ASAP.
  if (is_string($element_value) && strpos($element_value, '[') !== FALSE) {
    $element[$element_property] = \Drupal::token()->replace($element_value, $token_data, $token_options);
  }
}

Maybe this flag is needed in more places as well.