Skip transcoding when empty profiles were sent to B

[leszko]

This PR distinguishes between empty profiles and profiles not sent.

• no profiles {} => use default profiles
• empty profiles {"profiles":[]} => skip transcoding, only source is avaible

[codecov]

Codecov Report

Merging #2883 (d2acce4) into master (aa79cf5) will increase coverage by 0.00900%.
The diff coverage is 100.00000%.

@@                 Coverage Diff                 @@
##              master       #2883         +/-   ##
===================================================
+ Coverage   56.37255%   56.38155%   +0.00900%     
===================================================
  Files             89          89                 
  Lines          19380       19384          +4     
===================================================
+ Hits           10925       10929          +4     
  Misses          7849        7849                 
  Partials         606         606                 

Files	Coverage Δ
server/broadcast.go	77.61317% <100.00000%> (+0.05541%)	⬆️
server/mediaserver.go	67.22689% <100.00000%> (+0.02506%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aa79cf5...d2acce4. Read the comment docs.

Files	Coverage Δ
server/broadcast.go	77.61317% <100.00000%> (+0.05541%)	⬆️
server/mediaserver.go	67.22689% <100.00000%> (+0.02506%)	⬆️