[FIX] hr_holidays: validity constraint

- Create a leave type valid from June 1st
- Create a leave in May using the created leave type

Nothing prevents the user to do it.

It is because the constraint is implemented only if there is a validity
start and end date.

Fixes #32107
opw-2009800

closes odoo/odoo#34225

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>