Return a blank string instead of null

After fresh installing Kart on a Win7 machine, i had no settings input so testing for a custom background image path would throw a TypeError because path doesn't accept null arguments:

It would occur in [line 48 of the home controller](https://github.com/maddox/kart/blob/24a0b18f112b6c9f6e5ca80b7a45288ce9e4c760/app/src/controllers/home.coffee#L48):

```coffee
custom_background_path = path.join(@settings.romsPath(), 'background.png')
```

This patch returns a string instead of a null for a setting if the setting isn't in local storage.