fix: 4627 - no more nutrients wiped out

[monsieurtanuki]

What

• 2 bugs fixed in this PR.
• Minor fixed bug: there was a "cannot load" error that prevented the user from entering the nutrition facts page the very first time (then it was OK).
• The "Empty nutrition facts" bug is also fixed here:
  • when background tasks are piling up, we sent the request to the server and meanwhile we compute the local product as the initial product with all the local-and-not-server-committed changes on top.
  • the problem was that in those changes we overwrote existing nutrition facts with empty nutrition facts (from a product change that had no nutrition facts and it was transformed into existing but empty nutrition facts).

Fixes bug(s)

• Fixes: Nutrition input issues #4627

Impacted files

• ordered_nutrients_cache.dart: removed a bug I introduced in fix: 1815 - around context.mounted #4768.
• up_to_date_changes.dart: now nutriments are safer when background tasks are running.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b101d75) 9.67% compared to head (627f6bd) 9.67%.

Files	Patch %	Lines
...smooth_app/lib/data_models/up_to_date_changes.dart	0.00%	1 Missing ⚠️
...app/lib/pages/product/ordered_nutrients_cache.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #4902   +/-   ##
=======================================
  Coverage     9.67%   9.67%           
=======================================
  Files          318     318           
  Lines        16115   16115           
=======================================
  Hits          1559    1559           
  Misses       14556   14556           

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[g123k]

LGTM 👌

[teolemon]

Thanks @monsieurtanuki and @g123k