API-Platform: Set skip_null_values for all entites for apip 2.x

AppContextBuilder.php

/** 
* problem: skip_null_values can not be set for all entities in config. API-Platform 3 has this as default
*          
* solution: decorate api_platform.serializer.context_builder and set skip_null_values to false for all resources in one place:
*/
class AppContextBuilder implements SerializerContextBuilderInterface
{
  private $decorated;

  public function __construct(SerializerContextBuilderInterface $decorated)
  {
    $this->decorated = $decorated;
  }

  public function createFromRequest(Request $request, bool $normalization, ?array $extractedAttributes = null): array
  {
    $context = $this->decorated->createFromRequest($request, $normalization, $extractedAttributes);
    $context['skip_null_values'] = false;

    return $context;
  }
}