[FIX] stock: avoid traceback when changing operation type

Steps to reproduce the bug:
- create a new transfer
- Click on new line but leave it empty
- Change operation type

Problem:
A traceback is triggered, because the onchange of picking_type is
triggered, so we try to get the description of each product in each
move. However, since the move does not have a product set,
an error is thrown:
https://github.com/odoo/odoo/blob/16.0/addons/stock/models/product.py#L235-L239



opw-3349646

closes odoo/odoo#125010

Signed-off-by: William Henrotin (whe) <whe@odoo.com>