Fix support to denormalize plain object types

When denormalizing properties with a plain `object` type without a class the `ObjectNormalizer` would fail with a `TypeError`.

Author: andersonamuller

src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php

@@ -584,7 +584,7 @@ private function validateAndDenormalize(array $types, string $currentClass, stri
 
                 $expectedTypes[Type::BUILTIN_TYPE_OBJECT === $builtinType && $class ? $class : $builtinType] = true;
 
-                if (Type::BUILTIN_TYPE_OBJECT === $builtinType) {
+                if (Type::BUILTIN_TYPE_OBJECT === $builtinType && null !== $class) {
                     if (!$this->serializer instanceof DenormalizerInterface) {
                         throw new LogicException(sprintf('Cannot denormalize attribute "%s" for class "%s" because injected serializer is not a denormalizer.', $attribute, $class));
                     }

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

@@ -113,6 +113,17 @@ public function testDenormalizeWithExtraAttributesAndNoGroupsWithMetadataFactory
         );
     }
 
+    public function testDenormalizePlainObject()
+    {
+        $extractor = new PhpDocExtractor();
+        $normalizer = new ObjectNormalizer(null, null, null, $extractor);
+        $dummy = $normalizer->denormalize(['plainObject' => (object) ['foo' => 'bar']], DummyWithPlainObject::class);
+
+        $this->assertInstanceOf(DummyWithPlainObject::class, $dummy);
+        $this->assertInstanceOf(\stdClass::class, $dummy->plainObject);
+        $this->assertSame('bar', $dummy->plainObject->foo);
+    }
+
     public function testDenormalizeCollectionDecodedFromXmlWithOneChild()
     {
         $denormalizer = $this->getDenormalizerForDummyCollection();
@@ -598,6 +609,12 @@ protected function setAttributeValue(object $object, string $attribute, $value,
     }
 }
 
+class DummyWithPlainObject
+{
+    /** @var object */
+    public $plainObject;
+}
+
 class ObjectWithBasicProperties
 {
     /** @var bool */