When deleting all nutrients, some computed values are kept in DB

[raphael0202]

When we remove all nutrients from a product using the web version, we still have the following (computed values) in the nutriments field:

{
    "nutriments": {
        "carbon-footprint-from-known-ingredients_100g": 50.336,
        "carbon-footprint-from-known-ingredients_product": 252,
        "carbon-footprint-from-known-ingredients_serving": 27.2,
        "fruits-vegetables-legumes-estimate-from-ingredients_100g": 0,
        "fruits-vegetables-legumes-estimate-from-ingredients_serving": 0,
        "fruits-vegetables-nuts-estimate-from-ingredients_100g": 0,
        "fruits-vegetables-nuts-estimate-from-ingredients_serving": 0,
        "nova-group": 1,
        "nova-group_100g": 1,
        "nova-group_serving": 1,
        "nutrition-score-fr": -7,
        "nutrition-score-fr_100g": -7
    }
}

In Robotoff, I added a special case to consider the product nutrients as missing if there are only these keys: openfoodfacts/robotoff@0e4aff3.