Fix the shorthand access to another controller through needs

Without initiating the controller property, it is considered as a proxy access to the underlying object (in the case of an `ObjectController`) or array (in the case of an `ArrayController`). It results in an error in both cases:

```
    Uncaught TypeError: Object [object Object] has no method 'addArrayObserver'
```

```
    Uncaught Error: assertion failed: Cannot delegate set('auth',  <App.AuthController:ember340>) to the 'content' property of object proxy <App.IdeaController:ember447>: its 'content' is undefined.
```