[FIX] hr_holidays: Fix access error on leave allocation with multi-company

Current Behaviour:
As Admin:
- Install timeoff and enable Multi company
- Create new company and switch to that company
- Go to timeoff > Managers > All > Allocations > Create a record
An AccessError is raised from `_onchange_type` because it try to
access `employee_ids[:1].id` and it may gives employee of another company.

Expected Behaviour:

After this commit, it will not raise the AccessError. if employee of
that company is exist then and then set on leave allocation.
so we have already compute field `employee_id`, it will gives employee
of current company.

closes odoo/odoo#49120

Taskid: 2179571
Closes: #45381
X-original-commit: 73b3d11a0b443116ce91d889397c6d85a13ca1eb
Related: odoo/enterprise#9712
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/hr_holidays/models/hr_leave_allocation.py

@@ -308,7 +308,7 @@ class HolidaysAllocation(models.Model):
     def _onchange_type(self):
         if self.holiday_type == 'employee':
             if not self.employee_id:
-                self.employee_id = self.env.user.employee_ids[:1].id
+                self.employee_id = self.env.user.employee_id.id
             self.mode_company_id = False
             self.category_id = False
         elif self.holiday_type == 'company':
@@ -321,7 +321,7 @@ class HolidaysAllocation(models.Model):
             self.mode_company_id = False
             self.category_id = False
             if not self.department_id:
-                self.department_id = self.env.user.employee_ids[:1].department_id.id
+                self.department_id = self.env.user.employee_id.department_id.id
         elif self.holiday_type == 'category':
             self.employee_id = False
             self.mode_company_id = False