Take special care of movie rating values when they're not set.

"N/A" values or others that are parsed as NaN are saved as -1 in the store.
jshint fails when you use `==`, you should use `===`.

Apart from that, `Ember` has a helper function to validate empty values http://emberjs.com/api/classes/Ember.html#method_isEmpty

```js
if (Ember.isEmpty(imdbScore)) {
  return "width: 0";
} else { ... }
```