[webrtc-pc] Example 11 expects event.streams[0] from streamless tracks (#2217)

jan-ivar has just created a new issue for https://github.com/w3c/webrtc-pc:

== Example 11 expects event.streams[0] from streamless tracks ==
[Example 11](https://w3c.github.io/webrtc-pc/#advanced-peer-to-peer-example-with-warm-up) adds streamless tracks:
```js
  audio = pc.addTransceiver('audio');
  video = pc.addTransceiver('video');
```
which is fine, except it also expects `event.streams` to be non-zero in `pc.ontrack`, which it won't:
```js
  remoteView.srcObject = event.streams[0];
```

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2217 using your GitHub account