WebRTC Visualization problem on Azure ML notebooks / Jupyter [bug]

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hi!
I'm trying to get the `draw_webrtc.py` demo running on my Azure ML jupyter notebook.
It's running on a compute with a GPU, so that shouldn't be the problem.

When executing your example, I get the following ouput:
```
Jupyter environment detected. Enabling Open3D WebVisualizer.
[Open3D INFO] WebRTC GUI backend enabled.
[Open3D INFO] WebRTCWindowSystem: HTTP handshake server disabled.
[Open3D INFO] WebRTC GUI backend enabled.
[Open3D INFO] Window window_0 created.
[Open3D INFO] EGL headless mode enabled.
[Open3D INFO] ICE servers: {"stun:stun.l.google.com:1****2", "turn:user:password@**.**.**.**:3478", "turn:user:password@**.**.**.**:3478?transport=tcp"}
[Open3D INFO] Set WEBRTC_STUN_SERVER environment variable add a customized WebRTC STUN server.
[Open3D INFO] WebRTC Jupyter handshake mode enabled.
```

However I don't see anything appearing in my notebook. It's the same for jupyter, jupyter lab and the Azure notebook.
Sometimes I also get a
```
[Open3D INFO] Window window_1 created.
```

But again, nothing to see.

Any idea what I'm doing wrong here? Isn't it supposed to open a visualizer window inside the cell?