feature #28661 [Serializer] Add an option to skip null values (dunglas)

This PR was squashed before being merged into the 4.2-dev branch (closes #28661).

Discussion
----------

[Serializer] Add an option to skip null values

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      |no
| New feature?  | yes <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | todo

Adds a new option to not serialize `null` values:

```php
$dummy = new class {
    public $foo;
    public $bar = 'notNull';
};

$normalizer = new ObjectNormalizer();
$result = $normalizer->normalize($dummy, 'json', ['skip_null_values' => true]);
// ['bar' => 'notNull']
```

This feature is the only missing part to add [JSON Merge Patch](https://tools.ietf.org/html/rfc7386) support in [API Platform](https://api-platform.com).
It will also help supporting this RFC in all other projects using the Symfony Serializer.

Commits
-------

d3c5055 [Serializer] Add an option to skip null values

src/Symfony/Component/Serializer/CHANGELOG.md

@@ -4,6 +4,7 @@ CHANGELOG
 4.2.0
 -----
 
+ * added a `skip_null_values` context option to not serialize properties with a `null` values
  * `AbstractNormalizer::handleCircularReference` is now final and receives
    two optional extra arguments: the format and the context
  * added support for XML comment encoding (encoding `['#comment' => ' foo ']` results `<!-- foo -->`)

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

@@ -35,6 +35,7 @@ abstract class AbstractObjectNormalizer extends AbstractNormalizer
     const ENABLE_MAX_DEPTH = 'enable_max_depth';
     const DEPTH_KEY_PATTERN = 'depth_%s::%s';
     const DISABLE_TYPE_ENFORCEMENT = 'disable_type_enforcement';
+    const SKIP_NULL_VALUES = 'skip_null_values';
 
     private $propertyTypeExtractor;
     private $typesCache = array();
@@ -402,6 +403,10 @@ private function getTypes(string $currentClass, string $attribute)
      */
     private function updateData(array $data, string $attribute, $attributeValue, string $class, ?string $format, array $context): array
     {
+        if (null === $attributeValue && ($context[self::SKIP_NULL_VALUES] ?? false)) {
+            return $data;
+        }
+
         if ($this->nameConverter) {
             $attribute = $this->nameConverter->normalize($attribute, $class, $format, $context);
         }

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

@@ -161,6 +161,16 @@ public function testExtraAttributesException()
             'allow_extra_attributes' => false,
         ));
     }
+
+    public function testSkipNullValues()
+    {
+        $dummy = new Dummy();
+        $dummy->bar = 'present';
+
+        $normalizer = new ObjectNormalizer();
+        $result = $normalizer->normalize($dummy, null, array(AbstractObjectNormalizer::SKIP_NULL_VALUES => true));
+        $this->assertSame(array('bar' => 'present'), $result);
+    }
 }
 
 class AbstractObjectNormalizerDummy extends AbstractObjectNormalizer