take into account if schema = {} is same as if schema = true

jsonform.gadget.js

@@ -408,7 +408,8 @@
     // XXX `if then else` construction can be simplify to
     // anyOf(allOf(if_schema, then_schema), else_schema)
     // and realized by existed rails
-    if (schema === undefined) {
+    if (schema === undefined ||
+      Object.keys(schema).length === 0) {
       schema = true;
     }
     if (schema.anyOf !== undefined) {