[FIX] mrp: Scheduled date on empty workcenter cause traceback

Issue: When creating a manufacturing order and setting a scheduled date
when no workcenter is set, a traceback shows up

Steps to reproduce :
 1) Install Manufacturing
 2) Enable work centers
 3) Create a manufacturing order, in the work order tab, add a new line
 and set a Scheduled Start Date, confirm the date
 -> Traceback

opw-2633940

closes odoo/odoo#75992

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>