Allow line without party on payment

The payable or receivable account may not have party required set but it
should still be allowed to register payment their lines.

issue6787
review39701002

payment.py

@@ -139,7 +139,7 @@ class Payment(Workflow, ModelSQL, ModelView):
                 ['OR', ('credit', '>', 0), ('debit', '<', 0)],
                 ),
             ('account.kind', 'in', ['receivable', 'payable']),
-            ('party', '=', Eval('party', None)),
+            ('party', 'in', [Eval('party', None), None]),
             If(Eval('state') == 'draft',
                 [
                     ('reconciliation', '=', None),