Make editor audio-only proof

[Arnei]

Fixes #1636.

Make sure the editor deals reasonably well with audio-only tracks.

Includes:

• Displaying a little image for the player if there is no video stream.
• Make sure the track selection makes sense (don't display a video stream for an audio only track, don't allow users to deselect all tracks)
• Disable thumbnail generation for audio-only tracks, but keep thumbnail generation.

How to test this

The default community workflows attempt to circumvent the audio-only issue somewhat by adding dummy video track consisting of one image to the preview tracks for the editor. To generate true audio-only preview tracks for testing, you will want to change the relevant part in etc/workflows/partial-preview to something like this:

  # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  # Track previews

  # Create track previews for video editor stream selection feature
  # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  - id: image
    exception-handler-workflow: partial-error
    description: Create video preview images for presenter and presentation
    configurations:
      - source-flavor: '*/source'
      - target-flavor: '*/video+preview'
      - target-tags: preview
      - encoding-profile: editor.tracks.preview
      - time: 50%

  - id: prepare-av
    if: NOT (${presenter_source_video} OR ${presentation_source_video}) AND ${presenter_source_audio}
    description: Preparing presenter audio and video work versions
    configurations:
      - source-flavor: 'presenter/source'
      - target-flavor: 'presenter/preview'
      - rewrite: false
      - audio-muxing-source-flavors: '*/source'

  - id: prepare-av
    if: NOT (${presenter_source_video} OR ${presentation_source_video}) AND ${presentation_source_audio}
    description: Preparing presenter audio and video work versions
    configurations:
      - source-flavor: 'presentation/source'
      - target-flavor: 'presentation/preview'
      - rewrite: false
      - audio-muxing-source-flavors: '*/source'

  - id: tag
    description: Tagging media package elements
    configurations:
      - source-flavors: 'presenter/preview, presentation/preview'
      - target-tags: preview
      - copy: false

Make sure the editor still behaves reasonably for normal video files as well.

Screenshot:

[github-actions]

This pull request is deployed at test.editor.opencast.org/1706/2026-05-28_14-01-43/ .
It might take a few minutes for it to become available.