Replace wrong usage of False by None

(transplanted from 600c940612df5a8cd323fa167be20022e91e53bd)

invoice.py

@@ -2213,7 +2213,7 @@ class PayInvoiceAsk(ModelView):
     lines = fields.Many2Many('account.move.line', None, None, 'Lines',
         domain=[
             ('id', 'in', Eval('lines_to_pay')),
-            ('reconciliation', '=', False),
+            ('reconciliation', '=', None),
             ],
         states={
             'invisible': Eval('type') != 'writeoff',