[FIX] mrp: write one of start or end date on workorder

When writing a workorder start or end date from its manufacturing order
(and if the MO is unplanned), an error is thrown

Steps to reproduce:
1. Install Manufacturing
2. Go to Settings > Manufacturing > Operations and enable Work Orders
3. Open an MO with Work Orders
4. Unplan the MO
5. Add a Scheduled End Date to one of the Work Orders
6. Save
7. An error is thrown

Solution:
Compute the new duration only if both `start_date` and `end_date` are
set (and one of `date_planned_start` and `date_planned_finished` is
passed in values)

Problem:
When only one of `date_planned_start` or `date_planned_finished` is
written on an `mrp_workorder` (and the other is not set), the method
`_calculate_duration_expected` is called but it fails because it passes
a False value instead of a datetime which will raise the error `'bool'
object has no attribute 'tzinfo'`

opw-3004280

closes odoo/odoo#101961

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