Queue items become stuck if a recipient user is deleted

Steps to reproduce

1. Configure mass contact to use cron, so that the queues will be used.
2. Create a mass contact message.
3. Run the mass_contact_queue_messages queue to populate the mass_contact_send_message queue with items.
4. Delete one of the users who is a recipient of that message.
5. Run the mass_contact_send_message queue, and you'll (eventually) run into exception Call to a member function getEmail() on null in Drupal\mass_contact\MassContact->sendMessage().