[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#38784

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>

addons/purchase/models/purchase.py

@@ -179,7 +179,7 @@ class PurchaseOrder(models.Model):
         for line in new_po.order_line:
             if new_po.date_planned:
                 line.date_planned = new_po.date_planned
-            else:
+            elif line.product_id:
                 seller = line.product_id._select_seller(
                     partner_id=line.partner_id, quantity=line.product_qty,
                     date=line.order_id.date_order and line.order_id.date_order.date(), uom_id=line.product_uom)