[IMP] hr_holidays: use employee's partner to invite him to the event of his time-off

Events for time-off created with robodoo (demo data) wouldn’t invite the
partner related to a time off event as that partner didn’t have an user
counterpart. However, since he is an employee, he has a partner and can be
added to the event. This commit changes the check to fallback to the employee
partner if the user does not exist.

task-3455051
task-3389317

Part-of: odoo/odoo#138670

addons/hr_holidays/models/hr_leave.py

@@ -1025,9 +1025,9 @@ class HolidaysRequest(models.Model):
                 'res_id': holiday.id,
             }
             # Add the partner_id (if exist) as an attendee
-            if user and user.partner_id:
-                meeting_values['partner_ids'] = [
-                    (4, user.partner_id.id)]
+            partner_id = (user and user.partner_id) or (holiday.employee_id and holiday.employee_id.work_contact_id)
+            if partner_id:
+                meeting_values['partner_ids'] = [(4, partner_id.id)]
             result[user.id].append(meeting_values)
         return result