[FIX] delivery: avoid calling unknown method.

Only call `product_id_change_margin` method if exists. This method is
defined in `sale_margin` module, but as there is no dependency between
modules (nor a bridge module), the call have to be done here.

addons/delivery/models/sale_order.py

@@ -92,8 +92,13 @@ class SaleOrder(models.Model):
         }
         if self.order_line:
             values['sequence'] = self.order_line[-1].sequence + 1
-        so = SaleOrderLine.create(values)
-        so.product_id_change_margin()
+        sol = SaleOrderLine.create(values)
+        if hasattr(sol, 'product_id_change_margin'):
+            # if `sale_margin` module is installed, force computation of field `purchase_price`
+            # As there is not dependency between modules (nor a bridge module), this is the only
+            # place we can get the value for this field
+            sol.product_id_change_margin()
+        return sol
 
 
 class SaleOrderLine(models.Model):