Add constraint to check if track exists before trying to initialize it

Example of removed track's json:

```json
{"added_at"=>"2013-09-20T08:14:29Z", "added_by"=>{"external_urls"=>{"spotify"=>"http://open.spotify.com/user/1158639847"}, "href"=>"https://api.spotify.com/v1/users/1158639847", "id"=>"1158639847", "type"=>"user", "uri"=>"spotify:user:1158639847"}, "track"=>nil}
```