[FIX] hr_holidays: Don't set all employees absent

When computing `is_absent` for multiple employees
(multiple records in `self`), if one employee is really absent
(leave), all employee in the recordset are also considered
absent.

Note: it never happens in saas-12.2 but in master
and possibly custo.

closes odoo/odoo#34241

Signed-off-by: Romain Libert (rli) <rli@odoo.com>