[FIX] hr_holidays: Impossible to allocate future leaves

Steps to reproduce the bug:
- Let's consider Today = 31/10/2019
- 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

So now, when no default_date_from is in the context, no domain is applied

opw:2092830

closes odoo/odoo#39645

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