Don't throw needless exception

Closes #3443667

src/ReferenceUpgrader.php

@@ -29,15 +29,9 @@ final class ReferenceUpgrader {
    * Registers entities.
    */
   public function registerEntity(EntityInterface $entity) {
-    $entityId = $entity->id() ?? $this->throwNeedsId();
-    $this->entities[$entity->getEntityTypeId()][$entityId] = $entity;
-  }
-
-  /**
-   * Throws exceptions on trying to register entities without id.
-   */
-  private function throwNeedsId() {
-    throw new \RuntimeException("Can only register entity with ID.");
+    if ($entityId = $entity->id()) {
+      $this->entities[$entity->getEntityTypeId()][$entityId] = $entity;
+    }
   }
 
   /**