[FIX] sale_timesheet: Fallback on user employee when creating timesheet

Current behavior:
  If you set a specific sale_order_item for an employee on a project, and
  create a timesheet line from the "My Timesheets" menu,
  the sale_order_item will not be the one you selected for this employee.
  This happens because when you create a timesheet line from the "My
  Timesheets" menu, the employee is not set on the timesheet line.
  To fix this we fallback on the user's employee when creating a timesheet
  line from the "My Timesheets" menu.

Steps to reproduce:
  - Make sure you'r logged in as Mitchel Admin
  - Create a service product that create project and tasks
  - Create a sale order with this product
  - Go to the project and set a sale order item for Mitchel Admin in the
    invoicing tab
  - Go to the "My Timesheets" menu and click "Add Line"
  - Select the project you created
  - The sale order item will not be autofilled with the one set on the
    invoicing tab of the project.

opw-3463849

closes odoo/odoo#133478

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