Prevent exception if doesn't exist any payment_term.

issue3237
review917002
(transplanted from 64a62ed35a409e44326b9b7f93d0658ffedcb913)

opportunity.py

@@ -198,8 +198,9 @@ class SaleOpportunity(Workflow, ModelSQL, ModelView):
                     self.party.customer_payment_term.rec_name
         if not res['payment_term']:
             res['payment_term'] = self.default_payment_term()
-            res['payment_term.rec_name'] = PaymentTerm(
-                res['payment_term']).rec_name
+            if res['payment_term']:
+                res['payment_term.rec_name'] = PaymentTerm(
+                    res['payment_term']).rec_name
         return res
 
     def _get_sale_line_opportunity_line(self, sale):