Issue #3490858 by mfb, smustgrave: Align return type phpdoc for...

Issue #3490858 by mfb, smustgrave: Align return type phpdoc for EntityChangedInterface::getChangedTime() with EntityChangedTrait

(cherry picked from commit 811fe8a3)

core/lib/Drupal/Core/Entity/EntityChangedInterface.php

@@ -21,8 +21,9 @@ interface EntityChangedInterface extends EntityInterface {
   /**
    * Gets the timestamp of the last entity change for the current translation.
    *
-   * @return int
-   *   The timestamp of the last entity save operation.
+   * @return int|null
+   *   The timestamp of the last entity save operation. Some entities allow a
+   *   NULL value indicating the changed time is unknown.
    */
   public function getChangedTime();