[FIX] website_sale: empty error message

opw-1918450

Before this commit, the error message wasn't shown when a field was
empty.

Now, the error message shows correctly.

closes odoo/odoo#29993

addons/website_sale/controllers/main.py

@@ -539,7 +539,7 @@ class WebsiteSale(http.Controller):
             except ValidationError:
                 error["vat"] = 'error'
 
-        if [err for err in error.items() if err == 'missing']:
+        if [err for err in error.values() if err == 'missing']:
             error_message.append(_('Some required fields are empty.'))
 
         return error, error_message