fix: convert image to RGB before nutrition extraction

[raphael0202]

Fixes #1447

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 54.72%. Comparing base (750eea9) to head (ac307c5).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
robotoff/prediction/nutrition_extraction.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1448      +/-   ##
==========================================
- Coverage   54.74%   54.72%   -0.02%     
==========================================
  Files          85       85              
  Lines        8635     8637       +2     
==========================================
  Hits         4727     4727              
- Misses       3908     3910       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.