Check response is an array before converting to trams list

I discovered a crash in the TramTracker app. Occasionally, the API returns `responseObject: null` for the next trams list, rather than an array. Since we convert the array to a list of next trams, it crashes because it does not respond to selector `asNextTrams`.

I've added a quick check to make sure the response object is an array, and default to an empty array otherwise.