fix: issue where live dash streams without audio would hang

[mwaltza]

Description

Fixes this issue: https://github.com/videojs/http-streaming/issues/1471 Tested with this endpoint: https://livesim2.dashif.org/livesim2/segtimeline_1/WAVE/vectors/cfhd_sets/12.5_25_50/t1/2022-10-17/stream.mpd

• before the fix, this endpoint hangs after a few seconds
• after, this endpoint continues to play live video

Specific Changes proposed

Check whether media group is undefined before checking its contents

Requirements Checklist

• [x] Feature implemented / Bug fixed
• [ ] If necessary, more likely in a feature request than a bug fix
  • [ ] Unit Tests updated or fixed
  • [ ] Docs/guides updated
  • [ ] Example created (starter template on JSBin)
• [ ] Reviewed by Two Core Contributors