test(symfony): assert null default for jsonapi.use_iri_as_id

Config processing now returns null (default_null); the true
coercion happens in the extension. Companion to the deprecation
in this PR.

Author: soyuka

tests/Symfony/Bundle/DependencyInjection/ConfigurationTest.php

@@ -251,7 +251,7 @@ private function runDefaultConfigTests(array $doctrineIntegrationsToLoad = ['orm
                 'format' => 'jsonld',
             ],
             'jsonapi' => [
-                'use_iri_as_id' => true,
+                'use_iri_as_id' => null,
                 'allow_client_generated_id' => false,
             ],
             'enable_scalar' => true,