Handle missing media-stream silently

[tsundvoll]

Describe the bug
For the Placebots we don't have a media-stream available. Currently that leads to a error in the logs, but I think it should not be an error.

To Reproduce
Steps to reproduce the behavior:

1. Open Flotilla in dev or staging for Huldra
2. Open a previous mission for Placebot huldra
3. See in the console (F12) two error messages is logged:

Failed to GET /media-stream/3211b670-2723-434b-948e-c27e440544a4: 404 -
Error: No media config retrieved for robot with ID 3211b670-2723-434b-948e-c27e440544a4

Expected behavior
The error messages are not useful -> We don't expect any video streams for the Placebots.

Instead of returning 404 Not found; the backend can return 200 Success with [] empty payload; indicating that no media stream / config is available for this robot.

Screenshots