[IMP] delivery: allow to set delivery price from XML-RPC call

(a method that returns nothing is not callable through XML-RPC)

Backport of dc043cf1bb to 9.0

Fixes #12289 and tinyerp/erppeek#87

addons/delivery/models/sale_order.py

@@ -70,6 +70,8 @@ class SaleOrder(models.Model):
             else:
                 raise UserError(_('No carrier set for this order.'))
 
+        return True
+
     def _create_delivery_line(self, carrier, price_unit):
         SaleOrderLine = self.env['sale.order.line']