Fix implicit nullables

Closes #3499558

src/EventSubscriber/FieldStorageSubscriber.php

@@ -116,12 +116,12 @@ class FieldStorageSubscriber implements EventSubscriberInterface {
    *
    * @param string $entity_type_id
    *   The entity type ID.
-   * @param \Drupal\Core\Field\FieldStorageDefinitionInterface $field_storage_definition
+   * @param \Drupal\Core\Field\FieldStorageDefinitionInterface|null $field_storage_definition
    *   The field storage definition. It is only necessary to pass this if this
    *   a FieldStorageConfig object during presave and as such the definition is
    *   not yet available to the entity field manager.
    */
-  public function handleEntityType($entity_type_id, FieldStorageDefinitionInterface $field_storage_definition = NULL) {
+  public function handleEntityType($entity_type_id, ?FieldStorageDefinitionInterface $field_storage_definition = NULL) {
     $storage = $this->entityTypeManager->getStorage($entity_type_id);
     $der_fields = $this->entityFieldManager->getFieldMapByFieldType('dynamic_entity_reference');
     if ($field_storage_definition && ($field_storage_definition->getType() === 'dynamic_entity_reference')) {