Do not enforce empty shipment cost method for draft sale

As draft sale can be created by other modules, we must put such constraint
when quoting the sale.

Closes #11900

modules/sale_shipment_cost/sale.py

@@ -98,7 +98,7 @@ class Sale(metaclass=PoolMeta):
             ('shipment', "On Shipment"),
             ], "Shipment Cost Method",
         domain=[
-            If(~Eval('carrier'),
+            If(~Eval('carrier') & (Eval('state') != 'draft'),
                 ('shipment_cost_method', '=', None),
                 ()),
             ],