[FIX] stock_account: Cannot validate some invoices

Steps to reproduce the bug:

- create an invoice
- add one line without product but with a uom
- validate it

Bug:

A traceback was raised because no product was set on the invoice line.

opw:1825464

addons/stock_account/models/account_invoice.py

@@ -44,6 +44,8 @@ class AccountInvoiceLine(models.Model):
 
     def _get_anglo_saxon_price_unit(self):
         self.ensure_one()
+        if not self.product_id:
+            return self.price_unit
         return self.product_id._get_anglo_saxon_price_unit(uom=self.uom_id)
 
     def _get_price(self, company_currency, price_unit):