[FIX] sale_timesheet: Timesheet based invoicing does not work for subtasks

Steps to reproduce:

1. Enable Project Subtasks
2. On the "Office Design" Project, select a Task and create a subtask for it
3. Go to the Project Overview and Create Sales Order
4. Enter timesheets: 11 hours for the parent task, and 10 hours for the subtask
5.  On the created Sales Order check the invoiced time

Current behavior:
The 11 hours form the parent task are shown as delivered in the Sales Order, but the 10 hours from the subtask are not.

Expected behavior:
Both the parent ans subtask time should be shown as delivered, so that it can be invoices.

Closes #63930

opw:2426098

closes odoo/odoo#64791

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