[FIX] stock_account: compute price if no product

- Activate anglo-saxon accounting
- Create an invoice with a line without product but a UOM
- Validate

A traceback arises since `_compute_price` is called on an empty
recorset.

opw-1830042

addons/stock_account/models/product.py

@@ -201,8 +201,8 @@ class ProductProduct(models.Model):
     @api.model
     def _get_anglo_saxon_price_unit(self, uom=False):
         price = self.standard_price
-        if not uom or self.uom_id.id == uom.id:
-            return price
+        if not self or not uom or self.uom_id.id == uom.id:
+            return price or 0.0
         return self.uom_id._compute_price(price, uom)