[FIX] sale_timesheet: give no sol if sol in task is False

Before this commit, when the project is employee rate and the user sets
the sale order item to False in a task with this project, the linked
timesheets should have the sales order item to False, but if a
employee mapping is matched for this timesheet, the sales order item of
this mapping is given to the timesheet.

This commit checks if the sales order item in the task linked to the
timesheet and if no sales order item in the task then the sales order
item in the timesheet should also be empty (False).

closes #67055