[backend] fix group without a role must not have any capability (#5837)

[marieflorescontact]

Proposed changes

Related issues

• issue/5837

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (50613bb) 64.96% compared to head (ee5d8f5) 64.97%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5875   +/-   ##
=======================================
  Coverage   64.96%   64.97%           
=======================================
  Files         528      528           
  Lines       61999    62002    +3     
  Branches     4967     4972    +5     
=======================================
+ Hits        40279    40283    +4     
+ Misses      21720    21719    -1     

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