protected function normalizeFieldItems(FieldItemListInterface $field, $format, array $context) {

    $normalized_field_items = [];
    if (!$field->isEmpty()) {
      foreach ($field as $field_item) {
        $normalized_field_items[] = $this->serializer->normalize($field_item, $format, $context);
      }
    }
    return $normalized_field_items;
  }