Product sync: use default field values instead of None

[raphodn]

What

During the OxF daily sync, we create (or update) each product.
The input data is not always consistent, for instance some fields are sometimes missing. In that case, we currently set the field value to None.

We should rather skip that field, and let the model decide its default value.

Why

• fixes a major bug on the mobile : Duplicate proofs & prices sent to Open Prices smooth-app#5689
• better data quality
• Open Prices API : better match the specs

[raphodn]

Script to fix the Product array fields default values (list instead of None)

for f in Product.ARRAY_FIELDS:
    print(f)
    print(Product.objects.filter(**{f: None}).count())
    Product.objects.filter(**{f: None}).update(**{f: list()})

Script to delete duplicate prices (and proofs)

https://github.com/openfoodfacts/open-prices/wiki/Useful-scripts#delete-duplicate-prices