make sure a solution always shows up, use "demo" if exists

1-js/04-object-basics/01-object/3-is-empty/solution.md

@@ -1,10 +1 @@
 Just loop over the object and `return false` immediately if there's at least one property.
-
-```js
-function isEmpty(obj) {
-  for (let key in obj) {
-    return false;
-  }
-  return true;
-}
-```