fix: do not convert results that are already a JsonNode

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/jsonmigration/JsonMigrationHelper25.java

@@ -66,8 +66,8 @@ public JsonValue convertToJsonValue(Object object) {
   @SuppressWarnings("unchecked")
   @Override
   public JsonValue convertToClientCallableResult(JsonValue object) {
-    if (object == null) {
-      return null;
+    if (object == null || object instanceof JsonNode) {
+      return object;
     } else {
       switch (object.getType()) {
         case OBJECT: