Adding a default message for embargo notification block

[IAMlKeno]

As the embargoes module provides a notification message block.

By default the contact email is null and the message embargo_notification_message is excluded. When rendered, Drupal attempts to provide translatable markup. As null on embargo_notification_message is not a string, drupal return the following error message. Let's just provide a generic message.

InvalidArgumentException: $string ("") must be a string. in Drupal\Core\StringTranslation\TranslatableMarkup->__construct() (line 132 of /opt/www/drupal/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php).