Fix header style for empty cover images

Anime pages without a cover image are currently making a request to `/anime/null` because of [this](https://github.com/hummingbird-me/hummingbird/blob/1f1b00e640d71ad99c5704eed92eb91237dbc4d1/app/assets/javascripts/templates/anime.hbs#L1) line:

```html
<div class="series-header" {{bind-attr style=coverImageStyle}}>
```

```html
<div class="series-header" style="background-image: url('null'); background-position: 50% -0px;">
```

Disclosure: I don't know Ember (or JavaScript, for that matter). Is it okay to return `null` from `coverImageStyle` function, or does it need to return `""` instead?