FIX: products with a single price by qty may have empty price when adding it in a commercial piece

[jyhere]

How to reproduce the bug :

• set MAIN_FEATURE_LEVEL = 2
• Activate Price by quantity in Product module settings
• activate the ajax search product feature in Product module settings
• Activate Price by quantity in the product sell tab, add only one price by quantity (e.g for quantity 1).
• Create an order. In the product search box, search for the full product reference to get only 1 result (=> no option is shown is the select2 component).
• The price is 0.