refactor: #920 - when relevant, switched to "Navigator.push<void>"

[monsieurtanuki]

Impacted files:

• add_category_button.dart: now push<void> (no expected returned value, no pop'ed value from SimpleInputPage)
• add_ingredients_button.dart: now push<void> (no expected returned value, no pop'ed value from EditOcrPage)
• add_new_product_page.dart: added TODOs about problematic returned values
• add_nutrition_button.dart: now push<void> (no expected returned value, no pop'ed value from NutritionPageLoaded)
• edit_ingredients_page.dart: now poping nothing, as nobody cares about the value
• edit_product_page.dart: now push<void> (no expected returned value, no pop'ed value from AddBasicDetailsPage, EditOcrPage, NutritionPageLoaded, SimpleInputPage); added a TODO about a problematic returned value
• image_upload_card.dart: added a TODO about a problematic returned value
• login_page.dart: added a TODO about a problematic returned value
• new_product_page.dart now push<void> (no expected returned value, no pop'ed value from EditProductPage)
• product_image_gallery_view.dart: added a TODO about a problematic returned value
• product_list_page.dart: now push<void> (no expected returned value, no pop'ed value from PersonalizedRankingPage)
• product_title_card.dart: now push<void> (no expected returned value, no pop'ed value from AddBasicDetailsPage)
• question_page.dart: now push<void> (no expected returned value, no pop'ed value from LoginPage)
• scan_header.dart: now push<void> (no expected returned value, no pop'ed value from PersonalizedRankingPage)
• scan_product_card.dart: now push<void> (no expected returned value, no pop'ed value from ProductPage)
• smooth_product_card_found.dart: now push<void> (no expected returned value, no pop'ed value from ProductPage)
• smooth_product_card_not_found.dart: added a TODO about a problematic returned value
• summary_card.dart: now push<void> (no expected returned value, no pop'ed value from AddBasicDetailsPage, UserPreferencesPage, QuestionPage, `KnowledgePanelPage)

What

• We could not migrate to Navigator 2.0 / deep-linking because it looked like many times, we were not only opening pages but waiting for a returned value. And that is a deal breaker, unless flutter merges a specific PR (cf. Handle openfoodfacts.org links (Add deeplinking towards the product page) #920 (comment))
• This PR is about cleaning the code to detect where we actually need a returned value, and where we check a returned value for historical reasons but we don't have to anymore.
• That way we can focus on the cases where we do need a returned value. What should we do then? Probably change the "pages" into "dialogs", for which it's normal to return a value.
• There are now 8 cases in 6 files where it looks like we need a returned value. In 2 cases we probably could rewrite easily the code so that we don't.

Part of

• Handle openfoodfacts.org links (Add deeplinking towards the product page) #920
• Migration to Navigator 2.0 #1313

[M123-dev]

Good to have this checked, thanks @monsieurtanuki

[monsieurtanuki]

Thank you @M123-dev!