[FIX] account_payment_purchase: Don't propagate empty payment mode

Steps to reproduce the problem:

- Have a partner with not payment mode.
- Create a PO with such partner.
- No payment mode is filled.
- Now fill the payment mode in the partner.
- Create the invoice for the PO.

Current behavior:

The invoice has empty payment mode.

Expected behavior:

The invoice has the partner payment mode. Someone may think that having
no payment mode in the PO may prevail over the partner's payment mode,
or even their flows may consist in empyting the payment mode in the PO
for not binding it with anything yet, but that strategy has more holes
than the one implemented here, as the flow presented proves.

You can then use another one like having an extra payment mode
"Undetermined" or similar for doing such classification as a more
resilient strategy.

It includes a regression test that fails before the change and now is
correct.

TT38608

account_payment_purchase/models/__init__.py

@@ -1,3 +1,4 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
-from . import account_invoice, purchase_order
+from . import account_move
+from . import purchase_order

account_payment_purchase/models/account_move.py

@@ -1,5 +1,6 @@
 # Copyright 2016 Akretion (<http://www.akretion.com>).
-# Copyright 2017 Tecnativa - Vicent Cubells.
+# Copyright 2017 Tecnativa - Vicent Cubells
+# Copyright 2022 Tecnativa - Pedro M. Baeza
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
 from odoo import _, api, models
@@ -22,7 +23,8 @@ class AccountMove(models.Model):
                 "message": _("Selected purchase order have different payment mode."),
             }
             return res
-        self.payment_mode_id = new_mode
+        if new_mode:
+            self.payment_mode_id = new_mode
         if (
             self.invoice_partner_bank_id
             and new_bank
@@ -33,5 +35,6 @@ class AccountMove(models.Model):
                 "message": _("Selected purchase order have different supplier bank."),
             }
             return res
-        self.invoice_partner_bank_id = new_bank
+        if new_bank:
+            self.invoice_partner_bank_id = new_bank
         return res

account_payment_purchase/tests/test_account_payment_purchase.py

@@ -116,6 +116,15 @@ class TestAccountPaymentPurchase(SavepointCase):
             result and result.get("warning", {}).get("title", False), "Warning"
         )
 
+    def test_from_purchase_order_empty_mode_invoicing(self):
+        self.purchase.payment_mode_id = False
+        self.purchase.button_confirm()
+        invoice_form = Form(
+            self.env["account.move"].with_context(default_type="in_invoice")
+        )
+        invoice_form.purchase_id = self.purchase
+        self.assertEqual(invoice_form.payment_mode_id, self.payment_mode)
+
     def test_from_purchase_order_invoicing_bank(self):
         # Test partner_bank
         product = self.env["product.product"].create({"name": "Test product"})