[FIX] account_payment: properly compute destination account without partner

When the `partner_id` is not set (it is not required at the model
level), we can still compute the default destination account based on
the company chart of account configuration.

This is necessary to allow creating "customer" or "supplier" payments without
selecting a specific partner.

addons/account/models/account_payment.py

@@ -361,6 +361,12 @@ class account_payment(models.Model):
                 self.destination_account_id = self.partner_id.property_account_receivable_id.id
             else:
                 self.destination_account_id = self.partner_id.property_account_payable_id.id
+        elif self.partner_type == 'customer':
+            default_account = self.env['ir.property'].get('property_account_receivable_id', 'res.partner')
+            self.destination_account_id = default_account.id
+        elif self.partner_type == 'supplier':
+            default_account = self.env['ir.property'].get('property_account_payable_id', 'res.partner')
+            self.destination_account_id = default_account.id
 
     @api.onchange('partner_type')
     def _onchange_partner_type(self):