get first parent pictogram if not defined

[submarcos]

If sub category have no pictogram, pictogram returned by APi is this one available to first parent

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.60%. Comparing base (dd6a39e) to head (178e156).

Files	Patch %	Lines
backend/project/observations/models.py	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
- Coverage   81.75%   81.60%   -0.16%     
==========================================
  Files          19       19              
  Lines         466      473       +7     
  Branches       32       34       +2     
==========================================
+ Hits          381      386       +5     
- Misses         83       84       +1     
- Partials        2        3       +1     

Flag	Coverage Δ
python-unittests	81.60% <71.42%> (-0.16%)	⬇️

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