[FIX] stock: use default lang when no partner in deliveryslip

Step to reproduce:

1. create a product
2. duplicate it and change its translation (but DO NOT change the product name)
3. create a transfer for that new product without a contact
4. print delivery slip

Bug:
the db's product name is used instead of the translated name
(i.e. "original product name (copy)")

FIX:
when partner id is not set in picking, then default to the user's language

opw-3141202

closes odoo/odoo#116392

Signed-off-by: Tiffany Chang <tic@odoo.com>