fix(rtc-sender): Allow RTCSender to create tracks with zero InitialBitrate

[hanguyen-nuro]

Fix no rear / topdown camera.
Fix flickering due to fps.
Tested with playback on Nuro branch.

• Allow tracks with zero bitrate allocation to be created
• Keep zero-bitrate tracks active during runtime updates
• Use minimum 1000 bps for zero-bitrate track encoders
• Prevents connection loss for cameras with zero allocation
• Makes RTCSender behavior match NuroSender exactly

Fixes issues where cameras with 0 bitrate allocation would:

1. Fail to be created during initialization
2. Lose connection during runtime bitrate updates

[codecov]

Codecov Report

❌ Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.39%. Comparing base (96e4855) to head (d02034c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
sender/rtc_sender.go	30.76%	9 Missing ⚠️

❌ Your patch status has failed because the patch coverage (30.76%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   33.33%   33.39%   +0.06%     
==========================================
  Files          18       18              
  Lines        1542     1542              
==========================================
+ Hits          514      515       +1     
  Misses        981      981              
+ Partials       47       46       -1     

Flag	Coverage Δ
go	33.39% <30.76%> (+0.06%)	⬆️

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