Throw a more obvious error when the queue is empty on construction

[kevinsmith]

This provides clearer language for the error raised when an empty queue is passed into Relay on construction. Should fix #42.

@urugator and @niden, mind checking this one out?

[urugator]

I think it's fine. Similar error will be thrown if resolver returns non-middleware, so you may also consider adding is_callable($middleware) check with a better hint prior invoking the middleware:
https://github.com/relayphp/Relay.Relay/blob/2.x/src/Runner.php#L39