[Serializer] Fix anonymous test class

Author: alexandre-daubois

Tests/Normalizer/AbstractObjectNormalizerTest.php

@@ -855,7 +855,7 @@ public function testNormalizeWithIgnoreAnnotationAndPrivateProperties()
     public function testNormalizeBasedOnAllowedAttributes()
     {
         $normalizer = new class() extends AbstractObjectNormalizer {
-            protected function getAllowedAttributes($classOrObject, array $context, bool $attributesAsString = false)
+            protected function getAllowedAttributes($classOrObject, array $context, bool $attributesAsString = false): array
             {
                 return ['foo'];
             }
@@ -865,12 +865,12 @@ protected function extractAttributes(object $object, string $format = null, arra
                 return [];
             }
 
-            protected function getAttributeValue(object $object, string $attribute, string $format = null, array $context = [])
+            protected function getAttributeValue(object $object, string $attribute, string $format = null, array $context = []): mixed
             {
                 return $object->$attribute;
             }
 
-            protected function setAttributeValue(object $object, string $attribute, $value, string $format = null, array $context = [])
+            protected function setAttributeValue(object $object, string $attribute, $value, string $format = null, array $context = []): void
             {
             }
         };