[backend] Extract default value representative (#5348)

[SouadHadjiat]

Proposed changes

• extract default value representative, and fallback to id in any case

Related issues

• Error trying to set a default value for kill chain phases #5348

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e1c36f0) 65.66% compared to head (f249d50) 65.67%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5349   +/-   ##
=======================================
  Coverage   65.66%   65.67%           
=======================================
  Files         475      475           
  Lines       59277    59277           
  Branches     4330     4325    -5     
=======================================
+ Hits        38926    38929    +3     
+ Misses      20351    20348    -3     

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