fix: product_type not included when null

[monsieurtanuki]

Impacted files:

• product.dart: made most fields as "includeIfNull: false", including product_type
• product.g.dart: impact only on product_type

What

• The product field "product_type" was ALWAYS included in the "toJson" conversion, even if null.
• That had some problematic side effects on the server, that always saw a product_type provided for every "save product details" operation in Smoothie. And there's only one moment when this field should be sent: when we create the product. Afterwards, any save operation will be rejected if we populate the product_type, even with the same value.
• Now the product field "product_type" is only populated in the "toJson" conversion when it's not null, which only happens in Smoothie when we create the product in the correct OxF server.
• Beyond that, in order to avoid similar errors, a general code clean up should be performed around the "includeIfNull" clause for all classes, but that's another story.

Part of

• _Exception: Exception: Could not save product - API V2 - status=0 - errors=null smooth-app#5864

[teolemon]

👌 cc @stephanegigandet

[monsieurtanuki]

Thank you @teolemon for your review!