Fix producers can be null

[edenhaus]

Proposed Changes

/api/streams may have stream with null producers. If stream has only name with empty source list. This is useful for input stream.

go2rtc config:

streams:
  camera1:

Credits to @AlexxIT

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.89%. Comparing base (61087e4) to head (141d9fb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   97.85%   97.89%   +0.03%     
==========================================
  Files           7        7              
  Lines         280      285       +5     
  Branches       46       48       +2     
==========================================
+ Hits          274      279       +5     
  Misses          4        4              
  Partials        2        2              

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