fix(serializer): denormalize nullable collections of enums/objects

[soyuka]

Q	A
Branch?	4.3
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	Fixes #8379
License	MIT

Problem

Nullable arrays/collections of BackedEnums or objects fail to deserialize on POST/PATCH: the elements are left as raw strings instead of being denormalized into enum/object instances, which later triggers a Doctrine error such as:

Doctrine\ORM\Mapping\ReflectionEnumProperty::getValue(): Argument #1 ($item) must be of type BackedEnum, string given

Root cause

Symfony's TypeInfo represents a nullable collection value type as a NullableType wrapping the real ObjectType/BackedEnumType. For a nullable Doctrine SIMPLE_ARRAY column mapped with enumType, DoctrineExtractor produces list<Enum|null>|null, i.e. the collection value type is NullableType(ObjectType).

AbstractItemNormalizer::createAndValidateAttributeValue() checked $collectionValueType instanceof ObjectType directly. Since NullableType does not extend ObjectType, that check evaluated to false, the collection-of-objects branch was skipped, and the items were never handed off to the serializer for enum/object denormalization.

Fix

Unwrap the collection value type through WrappingTypeInterface before the instanceof ObjectType check, mirroring the existing unwrap loop already used later in the same method for the item type. Nested CollectionType wrapping is deliberately left untouched so a collection-of-collections is not misdetected as a collection-of-objects.

Tests

Added AbstractItemNormalizerTest::testDenormalizeNullableCollectionOfBackedEnums, which reproduces the NullableType(ObjectType) collection value type (as DoctrineExtractor emits it) and asserts the items are denormalized into enum instances. The test fails on 4.3 before the fix (items stay raw strings) and passes after.

Gates: php-cs-fixer clean, PHPStan clean on changed files, full src/Serializer/Tests suite green.