fix issue if schema_url undefined

jsonform.gadget.js

@@ -824,7 +824,7 @@
           z.schema = JSON.stringify(options.schema);
         }
       }
-      if (options.hasOwnProperty("schema_url")) {
+      if (options.schema_url) {
         z.schema_url = (new URL(options.schema_url, window.location))
           .toString();
       }