[FIX] stock: confusing error message

In case of a picking type where stock move lines are not shown, clicking
on 'Put in Pack' without 'Done' quantitites set shows a confusing error
message.

closes odoo/odoo#40928

X-original-commit: 76b72ab5
Related: odoo/enterprise#6906
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>