[16.0+]hr_timesheet_sheet: analytic lines are deleted changing the employee on sheet

[micheledic]

Module

hr_timesheet_sheet

Describe the bug

If you click on "New" on timesheet_sheet, you choose for example employee 1(this employee should have some analytic line), than you change the employee to "employee 2" without saving , the analytic lines of employee 1 are deleted

To Reproduce

Affected versions:
16.0
17.0
18.0(maybe)

Steps to reproduce the behavior:

1. Click on new
2. Choose an employee with analytic lines
3. Change employee without saving
4. Save
5. The lines of first employee are deleted

Expected behavior
The analytic lines shouldn't be deleted but only unlinked from sheet