[FIX] sale_project: confirm SO w/ milestones on SO with set project_id

Current behaviour:
If we confirm an SO with a service with a delivery based on
milestones, that creates both a project & task, and we set a project
on the SO, the project for the milestone should be the one set on
the SO, a ValidationError is raised for missing project on milestone.

Expected behaviour:
No ValidationError, the milestone should use the project set on the SO.

Steps to reproduce:
- Install Sales, Project
- Check Milestones in Settings
- Create a new project.
- Create a new product, type service, delivery milestones, on
  confirm create project & task
- Create an SO, add a line with the new product.
- Set a our new project on the SO (In the "Other Info" tab)
- Confirm the SO, you are met with a ValidationError.

Reason for the problem:
Since we are not *generating* a new project, `project_id` on the
sale_order_line is empty, and we base ourself on it when creating
the milestone in `_generate_milestone`, leading to the non-null
validation error, because this field is `required=True` on milestones.

Fix:
When creating the milestone in `_generate_milestone`, if the line
doesn't have a `project_id`, we take the one set on the order.
We don't set the `project_id` to the *determined_project*
(`_determine_project`) at line 305
(after: `so_line._timesheet_create_task(project=project)`) because
the `project_id` on the sale_order_line is marked as "Generated
Project", which is not true in our case (the project isn't generated
by the sale order line).

Affected versions:
- 16.0
- saas-16.1
- saas-16.2
- master

opw-3236235

closes odoo/odoo#120434

Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>