media_engine: Properly return codec when not negotiated

[haaspors]

Also added a test I found in Pion, however not really a test this PR affects.

It should be said that this fix is needed for RtpReceiver to work properly when using ORTC API.

[codecov]

Codecov Report

Attention: Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 61.35%. Comparing base (4bb9614) to head (523803d).

Files	Patch %	Lines
webrtc/src/api/media_engine/media_engine_test.rs	22.22%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #595      +/-   ##
==========================================
- Coverage   61.35%   61.35%   -0.01%     
==========================================
  Files         532      532              
  Lines       49715    49720       +5     
  Branches    12601    12619      +18     
==========================================
+ Hits        30504    30506       +2     
+ Misses       9868     9866       -2     
- Partials     9343     9348       +5     

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