[backend] fix internal_id being removed during observable fullsync (#8986)

[JeremyCloarec]

Proposed changes

• internal_id was being upserted to an undefined value in the case of fullsync. We now only set an internal_id in addStixCyberObservable if the internal_id is not undefined

Related issues

• [livestream] renaming of observable creates a new one in perfect sync mode #8986

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.22%. Comparing base (dde5551) to head (4fe30e9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9218      +/-   ##
==========================================
- Coverage   65.22%   65.22%   -0.01%     
==========================================
  Files         630      630              
  Lines       60199    60201       +2     
  Branches     6741     6744       +3     
==========================================
- Hits        39267    39266       -1     
- Misses      20932    20935       +3     

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[lndrtrbn]

Could it be possible to have a case where internal_id from the input is undefined or null and we want to keep this null value ? Or it is sure that in any case we don't want it ?

[SouadHadjiat]

I don't undesrtand in which case we would have an internal_id in the observable input, it's not part of the schema. And internal_id is not part of the stream, right ? @JeremyCloarec