Raise validation error when undefined 'completed_by' met

[niklub]

No description provided.

[codecov]

Codecov Report

Merging #1188 (6221859) into master (d024670) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1188   +/-   ##
=======================================
  Coverage   73.81%   73.82%           
=======================================
  Files         115      115           
  Lines        7731     7732    +1     
=======================================
+ Hits         5707     5708    +1     
  Misses       2024     2024           

Impacted Files	Coverage Δ
label_studio/tasks/serializers.py	78.32% <100.00%> (+0.06%)	⬆️

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