Use quantity set on the line to compute unit price from Shopify order

The quantity of the line item may be different if part of the line item has
been refunded or cancelled.

Closes #14103

modules/web_shop_shopify/sale.py

@@ -373,9 +373,9 @@ class Line(IdentifierMixin, metaclass=PoolMeta):
         total_discount = sum(
             Decimal(d.amount) for d in line_item.discount_allocations)
         unit_price = ((
-                (Decimal(line_item.price) * line_item.quantity)
+                (Decimal(line_item.price) * (quantity or 1))
                 - Decimal(total_discount))
-            / line_item.quantity)
+            / (quantity or 1))
         unit_price = round_price(Tax.reverse_compute(
                 unit_price, line.taxes, sale.sale_date))
         if line.product: