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.
If we store `null` in `firebase` we can validate if the value is empty so we avoid using magic constants (I know it was my idea but now i'm not sure)

```js
  tomatoBarStyle: function() {
    var tomatoScore = this.get('tomatoScore');

    return 'width: %@%'.fmt(Ember.isEmpty(tomatoScore) ? 0 : tomatoScore);
  }.property('tomatoScore'),
```