metrics: populate undetected audio/video codecs as "n/a"

[emranemran]

Several audio-less video is uploaded to livepeer. These jobs get recorded in the db as having a blank codec field which makes it tricky to parse stats. Explicitly set these job fields to "n/a" whenever a codec is missing for a given video/audio track.

[codecov]

Codecov Report

Merging #564 (9de9300) into main (bf5f658) will increase coverage by 0.03921%.
The diff coverage is 80.00000%.

@@                 Coverage Diff                 @@
##                main        #564         +/-   ##
===================================================
+ Coverage   54.75883%   54.79804%   +0.03921%     
===================================================
  Files             48          48                 
  Lines           3877        3887         +10     
===================================================
+ Hits            2123        2130          +7     
- Misses          1584        1585          +1     
- Partials         170         172          +2     

Impacted Files	Coverage Δ
pipeline/coordinator.go	80.71625% <80.00000%> (-0.30358%)	⬇️

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Impacted Files	Coverage Δ
pipeline/coordinator.go	80.71625% <80.00000%> (-0.30358%)	⬇️