Avoid error when emiting an AMQP job event when the job does not exist

[Martchus]

• Ignore non-existing jobs when trying to emit an AMQP event
  • This can happen when the job has been deleted meanwhile, e.g. via the API or during cleanup. It makes no sense to emit e.g. a job_done event when the job has already been deleted anyway.
  • This can also happen on concurrent deletions where one event is enough.
• Treat non-existing jobs similar to non-existing comments which we already ignore
• Avoid error observed in https://progress.opensuse.org/issues/178126

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.01%. Comparing base (6b8160f) to head (b7f4413).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6243   +/-   ##
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  Coverage   99.01%   99.01%           
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  Files         398      398           
  Lines       40017    40017           
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  Hits        39623    39623           
  Misses        394      394           

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