[backend] Fix null for objectParticipant in Task (#9752)

[Kedae]

Closes #9752

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.39%. Comparing base (2941a40) to head (0fa94e8).
Report is 30 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9755      +/-   ##
==========================================
+ Coverage   65.24%   65.39%   +0.14%     
==========================================
  Files         630      630              
  Lines       60315    60427     +112     
  Branches     6770     6887     +117     
==========================================
+ Hits        39354    39515     +161     
+ Misses      20961    20912      -49     

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[Archidoit]

Error when trying to add a participant :

[SarahBocognano]

Same when trying to build frontend, the type [Participant!] and [Participant!]! are in conflict. I wasn't able to try it :(

[Kedae]

Ready for a new review