rtmp: fix panic when publishing audio-only streams (#1459)

[aler9]

Fixes #1459

[codecov-commenter]

Codecov Report

Merging #1502 (549684c) into main (18a184c) will increase coverage by 0.19%.
The diff coverage is 100.00%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main    #1502      +/-   ##
==========================================
+ Coverage   65.58%   65.77%   +0.19%     
==========================================
  Files         118      118              
  Lines       12988    12988              
==========================================
+ Hits         8518     8543      +25     
+ Misses       3797     3771      -26     
- Partials      673      674       +1     

Impacted Files	Coverage Δ
internal/rtmp/conn.go	64.59% <100.00%> (+0.94%)	⬆️
internal/core/path_manager.go	91.36% <0.00%> (-0.67%)	⬇️
internal/core/rtmp_server.go	86.30% <0.00%> (-0.60%)	⬇️
internal/core/hls_muxer.go	50.94% <0.00%> (-0.48%)	⬇️
internal/core/webrtc_conn.go	54.85% <0.00%> (-0.33%)	⬇️
internal/core/rtmp_conn.go	67.34% <0.00%> (-0.21%)	⬇️
internal/core/hls_server.go	76.63% <0.00%> (+1.22%)	⬆️
internal/hls/client_downloader_primary.go	41.23% <0.00%> (+2.06%)	⬆️
internal/hls/client_processor_mpegts.go	64.39% <0.00%> (+2.27%)	⬆️
... and 1 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[aler9]

added in v0.21.6