[FIX] purchase: Impossible to duplicate a PO

Steps to reproduce the bug:
- Create a RFQ
- Add a line with a product
- Add a line with a note and save
- Duplicate the RFQ

Bug:

A traceback was raised because the function _select_seller was called on a line
with no product.

opw:2085977

closes odoo/odoo#38921

X-original-commit: 4ffc517d
Signed-off-by: Simon Goffin (sig) <sig@openerp.com>