[FIX] account: due date

On an invoice:
- Select a partner with payment term defined => due date D1 is set
- Change to a partner withour payment term defined

The due date D1 is kept.

opw-772063

addons/account/models/account_invoice.py

@@ -505,6 +505,7 @@ class AccountInvoice(models.Model):
 
         self.account_id = account_id
         self.payment_term_id = payment_term_id
+        self.date_due = False
         self.fiscal_position_id = fiscal_position
 
         if type in ('in_invoice', 'out_refund'):