send null if layout config is not configured

application/src/main/java/org/thingsboard/server/controller/MobileAppController.java

@@ -168,7 +168,7 @@ public class MobileAppController extends BaseController {
                     .collect(Collectors.toList());
             return JacksonUtil.toJsonNode(JacksonUtil.writeValueAsViewIgnoringNullFields(mobilePages, Views.Public.class));
         } else {
-            return JacksonUtil.newArrayNode();
+            return null;
         }
     }