Authz-client: fix ClassCast Exception when getting resource permissions

(#27483)

Signed-off-by: Clemens Zagler <c.zagler@noi.bz.it>

authz/client/src/main/java/org/keycloak/authorization/client/resource/AuthorizationResource.java

@@ -86,6 +86,7 @@ public class AuthorizationResource {
 
         if (request.getMetadata() == null) {
             metadata = new AuthorizationRequest.Metadata();
+            request.setMetadata(metadata);
         } else {
             metadata = request.getMetadata();
         }