[FIX] hr_holidays: Impossible to allocate future leave type

Steps to reproduce the bug:

- Let's consider Today = 21/11/2019 and Employee E
- Create a new hr.leave.type LT and set a validity from 01/01/2020 to 31/12/2020
- Set mode = Free Allocation Request and Validation = No Validation
- Try to create leave allocations for LT

Bug:

It was impossible to create a leave allocation for LT because Today < 01/01/2020
So it was impossible to allocate future leave.
We had to wait the 01/01/2020 to make the allocation of LT leaves

opw:2126300

closes odoo/odoo#40629

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>