Remove return type for Twig function workflow_metadata()

Technically it is allowed to have metadata other than string, even data that does not cast to string, like an array.
Also see symfony/symfony#36391

Extension/WorkflowExtension.php

@@ -117,7 +117,7 @@ public function getMarkedPlaces($subject, $placesNameOnly = true, $name = null)
      *                                                Use a string (the place name) to get place metadata
      *                                                Use a Transition instance to get transition metadata
      */
-    public function getMetadata($subject, string $key, $metadataSubject = null, string $name = null): ?string
+    public function getMetadata($subject, string $key, $metadataSubject = null, string $name = null)
     {
         return $this
             ->workflowRegistry