parent_path optional argument in getJsonPath()

jsonform/gadget_json_generated_form_child.js

@@ -1830,7 +1830,10 @@
     .declareMethod('getJsonPath', function (parent_path) {
       return this.parentGetJsonPath(this.element.getAttribute("data-json-parent"))
         .push(function (p) {
-          return p + parent_path;
+          if (parent_path) {
+            p = p + parent_path;
+          }
+          return p;
         });
     })