chore: ProductQuery.getLanguage() is now not nullable

[monsieurtanuki]

What

• One of the first things we do in the app init is setting the language. There are no accidental cases or typical use cases when the language is supposed to be null.
• For a better understanding of the app, it's better to make this method explicitly return a non null value.

Part of

• Make it possible to view the product name in several languages #3863

[codecov-commenter]

Codecov Report

Merging #3896 (adb4311) into develop (85a3525) will not change coverage.
The diff coverage is 9.09%.

@@           Coverage Diff            @@
##           develop    #3896   +/-   ##
========================================
  Coverage    10.68%   10.68%           
========================================
  Files          270      270           
  Lines        13428    13428           
========================================
  Hits          1435     1435           
  Misses       11993    11993           

Impacted Files	Coverage Δ
..._app/lib/pages/product/add_basic_details_page.dart	0.00% <0.00%> (ø)
...kages/smooth_app/lib/pages/product/ocr_helper.dart	0.00% <0.00%> (ø)
...app/lib/pages/product/ordered_nutrients_cache.dart	0.00% <0.00%> (ø)
...p/lib/pages/product/simple_input_page_helpers.dart	0.00% <0.00%> (ø)
...app/lib/pages/product/simple_input_text_field.dart	0.00% <0.00%> (ø)
...ges/smooth_app/lib/pages/product/summary_card.dart	0.00% <0.00%> (ø)
...ages/smooth_app/lib/query/paged_product_query.dart	8.33% <ø> (ø)
.../smooth_app/lib/query/product_questions_query.dart	0.00% <0.00%> (ø)
packages/smooth_app/lib/query/questions_query.dart	0.00% <0.00%> (ø)
packages/smooth_app/lib/query/product_query.dart	8.10% <50.00%> (ø)

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[g123k]

LGTM 👍

[monsieurtanuki]

Thank you @g123k for your review!