[Serializer] Fix denormalizing of array with empty body

This happens for example with XML empty tags denormalizing to an object array attribute.

Normalizer/AbstractObjectNormalizer.php

@@ -16,6 +16,7 @@
 use Symfony\Component\PropertyInfo\PropertyTypeExtractorInterface;
 use Symfony\Component\PropertyInfo\Type;
 use Symfony\Component\Serializer\Encoder\JsonEncoder;
+use Symfony\Component\Serializer\Encoder\XmlEncoder;
 use Symfony\Component\Serializer\Exception\ExtraAttributesException;
 use Symfony\Component\Serializer\Exception\LogicException;
 use Symfony\Component\Serializer\Exception\NotNormalizableValueException;
@@ -411,6 +412,10 @@ private function validateAndDenormalize(string $currentClass, string $attribute,
                 $data = [$data];
             }
 
+            if (XmlEncoder::FORMAT === $format && '' === $data && Type::BUILTIN_TYPE_ARRAY === $type->getBuiltinType()) {
+                return [];
+            }
+
             if (null !== $collectionValueType && Type::BUILTIN_TYPE_OBJECT === $collectionValueType->getBuiltinType()) {
                 $builtinType = Type::BUILTIN_TYPE_OBJECT;
                 $class = $collectionValueType->getClassName().'[]';

Tests/Normalizer/Features/ObjectDummy.php

@@ -5,6 +5,9 @@
 class ObjectDummy
 {
     protected $foo;
+    /**
+     * @var array
+     */
     public $bar;
     private $baz;
     protected $camelCase;

Tests/Normalizer/ObjectNormalizerTest.php

@@ -165,6 +165,19 @@ public function testDenormalize()
         $this->assertTrue($obj->isBaz());
     }
 
+    public function testDenormalizeEmptyXmlArray()
+    {
+        $normalizer = $this->getDenormalizerForObjectToPopulate();
+        $obj = $normalizer->denormalize(
+            ['bar' => ''],
+            ObjectDummy::class,
+            'xml'
+        );
+
+        $this->assertIsArray($obj->bar);
+        $this->assertEmpty($obj->bar);
+    }
+
     public function testDenormalizeWithObject()
     {
         $data = new \stdClass();