[FIX] hr_holidays: remove time off type if no employee_id

Steps to reproduce:
-------------------
- go to the hr leave form;
- select "By Employee" mode;
- select an employee-based leave type;
(example: Paid Time Off (20 remaining out of 20 days))
- change mode with "By Company";

Issue:
------
A specific leave to one employee can be validated
for all employees of a company.

Cause:
------
The `_compute_from_employee_id` method does not set
the `holiday_status_id` field to `False` in this case.

Solution:
---------
When we detect an `employee_id` change and it is `False`,
we must remove the current `holiday_status_id`.

opw-3386332

closes odoo/odoo#129653

X-original-commit: a7a8e78103962844d7d3752d41f9ca4b22165569
Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>
Signed-off-by: Thomas Lefebvre (thle) <thle@odoo.com>

addons/hr_holidays/models/hr_leave.py

@@ -455,7 +455,7 @@ class HolidaysRequest(models.Model):
             holiday.manager_id = holiday.employee_id.parent_id.id
             if holiday.holiday_status_id.requires_allocation == 'no':
                 continue
-            if len(holiday.employee_ids) > 1:
+            if not holiday.employee_id or len(holiday.employee_ids) > 1:
                 holiday.holiday_status_id = False
             elif holiday.employee_id.user_id != self.env.user and holiday._origin.employee_id != holiday.employee_id:
                 if holiday.employee_id and not holiday.holiday_status_id.with_context(employee_id=holiday.employee_id.id).has_valid_allocation: