cost_line could be empty dictionary if there is no cost

(transplanted from ebb11dd2659394e1fca65c231a3647c1a4647d76)

sale.py

@@ -154,7 +154,7 @@ class Sale(Model):
             values['lines'] = []
         for line in values['lines']:
             if line.get('shipment_cost'):
-                if line['shipment_cost'] != cost_line['shipment_cost']:
+                if line['shipment_cost'] != cost_line.get('shipment_cost'):
                     to_remove = line['id']
                     values['lines'].remove(line)
                     if cost_line: