fix: issue where live dash streams without audio would hang

[mwaltza]

Description

Fixes this issue: #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

• 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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.36%. Comparing base (7083c5e) to head (6f69756).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1524      +/-   ##
==========================================
- Coverage   86.40%   86.36%   -0.04%     
==========================================
  Files          43       43              
  Lines       11144    11144              
  Branches     2545     2545              
==========================================
- Hits         9629     9625       -4     
- Misses       1515     1519       +4     

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