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;
-}
-```

1-js/04-object-basics/01-object/8-multiply-numeric/solution.md

@@ -1,19 +0,0 @@
-``` js run
-
-// before the call
-let menu = {
-  width: 200,
-  height: 300,
-  title: "My menu"
-};
-
-function multiplyNumeric(obj) {
-  for (let key in obj) {
-    if (typeof obj[key] == 'number') {
-      obj[key] *= 2;
-    }
-  }
-}
-
-alert(menu); 
-```