[FIX] purchase : Email reminder crash

Current behavior:
When creating a PO with a product and a note you had a traceback when sending reminder and clicking on "No, Update dates"

Steps to reproduce:
1. Create a PO with one line and one note
2. Confirm it
3. Click on action>Send Reminder
4. Go to the mail and click "No" => Error

opw-2709323

closes odoo/odoo#81466

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