[FIX] hr_holidays: display allocation dates in the report

Steps to reproduce:
- go to "Reporting" (by Type) in the Time Off app.

Issue:
The start and end dates for the validity of allocations
are not displayed.
Furthermore, if a filter is applied on the date,
the allocations will not be taken into account

Cause:
The start and end dates are set to null in the query
that fetches records for the report.

opw-3143938

closes odoo/odoo#111036

Signed-off-by: Kevin Baptiste <kba@odoo.com>

addons/hr_holidays/report/hr_leave_report.py

@@ -59,8 +59,8 @@ class LeaveReport(models.Model):
                     allocation.holiday_status_id as holiday_status_id,
                     allocation.state as state,
                     allocation.holiday_type,
-                    null as date_from,
-                    null as date_to,
+                    allocation.date_from as date_from,
+                    allocation.date_to as date_to,
                     'allocation' as leave_type,
                     allocation.employee_company_id as company_id
                 from hr_leave_allocation as allocation