Issue #3506674: Add NULL return type to getAssociatedEntity().

Closes #3506674

modules/message_recipient_entity/src/MessageAssociatedEntityTrait.php

@@ -25,10 +25,10 @@ trait MessageAssociatedEntityTrait {
    * @param \Drupal\message\MessageInterface $message
    *   The message entity.
    *
-   * @return \Drupal\Core\Entity\ContentEntityInterface
-   *   The associated entity
+   * @return \Drupal\Core\Entity\ContentEntityInterface|null
+   *   The associated entity or NULL.
    */
-  public function getAssociatedEntity(MessageInterface $message): ContentEntityInterface {
+  public function getAssociatedEntity(MessageInterface $message): ?ContentEntityInterface {
     return $message->get('message_associated_entity')
       ?->first()
       ?->get('entity')