bug symfony#54505 [Serializer] initialize serializer in trait with null (xabbuh)

This PR was merged into the 7.0 branch.

Discussion
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[Serializer]  initialize serializer in trait with null

| Q             | A
| ------------- | ---
| Branch?       | 7.0
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        | Fix symfony#54417
| License       | MIT

Commits
-------

dae3726 initialize serializer in trait with null

Author: chalasr

src/Symfony/Component/Serializer/SerializerAwareTrait.php

@@ -16,7 +16,7 @@
  */
 trait SerializerAwareTrait
 {
-    protected SerializerInterface $serializer;
+    protected ?SerializerInterface $serializer = null;
 
     public function setSerializer(SerializerInterface $serializer): void
     {

src/Symfony/Component/Serializer/Tests/Normalizer/ObjectNormalizerTest.php

@@ -885,6 +885,16 @@ public function testSamePropertyAsMethodWithMethodSerializedName()
 
         $this->assertSame($expected, $this->normalizer->normalize($object));
     }
+
+    public function testNormalizeWithoutSerializerSet()
+    {
+        $normalizer = new ObjectNormalizer(new ClassMetadataFactory(new AttributeLoader()));
+
+        $this->expectException(LogicException::class);
+        $this->expectExceptionMessage('Cannot normalize attribute "foo" because the injected serializer is not a normalizer.');
+
+        $normalizer->normalize(new ObjectConstructorDummy([], [], []));
+    }
 }
 
 class ProxyObjectDummy extends ObjectDummy