JMS Serializer doesn't expose one property

You can serialize nulls as empty strings. Guess it help you a bit

$context = new SerializationContext();
$context->setSerializeNull(true);
$objectData = $serializer->serialize($object, 'json', $context);

For FOSRestBundle you can define it in settings

fos_rest:
    view:
        serialize_null: true

forgottenbas'es solution for FOSRestBundle didn't work for me. I have found a solution here https://github.com/FriendsOfSymfony/FOSRestBundle/pull/480

Use serializer section in your config, not view:

fos_rest:
    serializer:
        serialize_null: true