[FIX] hr_holidays: employee onchange when no employee set

When changing the employee to None in an allocation,
this onchange raised an error because it attempted
to set an employee browser record (empty but still)
in a department many2one.

- Create a new leave allocation
- Try to change the field Mode (holiday_type) from "By Employee" to "By Employee Tag"

Fixes #23345
opw-1820069

addons/hr_holidays/models/hr_holidays.py

@@ -216,7 +216,7 @@ class Holidays(models.Model):
     @api.onchange('employee_id')
     def _onchange_employee_id(self):
         self.manager_id = self.employee_id and self.employee_id.parent_id
-        self.department_id = self.employee_id and self.employee_id.department_id
+        self.department_id = self.employee_id.department_id
 
     @api.multi
     @api.depends('number_of_days_temp', 'type')