Missing langcode causes send to fail without actionable information

The scenario is as follows: One of our clients reported the send batch failing with a 500 error. Watchdog reported:

InvalidArgumentException: Invalid translation language () specified. in Drupal\Core\Entity\ContentEntityBase->getTranslation() (Zeile 873 in /var/www/htdocs/im.nrw/docroot/docroot/core/lib/Drupal/Core/Entity/ContentEntityBase.php).

Turns out somehow one of the subscribers in the DB had a null langcode (probably through a faulty import, it's a really old entry).

Anyway, I have attached a small patch that checks if a subscriber happens to have an empty langcode. If the langcode is empty, the subscriber is skipped during the batch and a message with a link to the faulty subscriber is written to watchdog.