[FIX] sale: do not always overwrite discount on soline

Before this commit, the discount on a so line was always brought to zero when changing the price

After this commit, we do this only when the pricelist is supposed to compute the discount

OPW 1867262
closes #25851

addons/sale/models/sale.py

@@ -1007,13 +1007,13 @@ class SaleOrderLine(models.Model):
 
     @api.onchange('product_id', 'price_unit', 'product_uom', 'product_uom_qty', 'tax_id')
     def _onchange_discount(self):
-        self.discount = 0.0
         if not (self.product_id and self.product_uom and
                 self.order_id.partner_id and self.order_id.pricelist_id and
                 self.order_id.pricelist_id.discount_policy == 'without_discount' and
                 self.env.user.has_group('sale.group_discount_per_so_line')):
             return
 
+        self.discount = 0.0
         product = self.product_id.with_context(
             lang=self.order_id.partner_id.lang,
             partner=self.order_id.partner_id.id,