Steps to reproduce
==================
- Create a blanket order
- Choose a vendor
- Add a product P
- Set a price
- Save and Confirm
- Click on the RFQs/Orders smart button
- Create a new RFQ
- Delete the line
- Add a new line with the same product P
- Save
-> A validation error appears because the date_planned is not set
Cause of the issue
==================
`_compute_price_unit_and_date_planned_and_name` is overriden and passes
an empty recordset to the super method.
Solution
========
We can compute the `date_planned` using the selected seller
opw-3416573
closesodoo/odoo#132428
X-original-commit: d7db1d799fe042ef248968c6dee53eaf6e0ed3db
Signed-off-by: Tiffany Chang (tic) <tic@odoo.com>