[FIX] hr_work_entry_holidays: Split existing leave when opening a contract

Purpose
=======

Currenlty an error is raise saying that a leave cannot be set accross
overlapping contracts.

But in this special case we could split the time off and allow this simple
action.

Taskid: 2836901
X-original-commit: 1012a7ea8033675b6ebab96922e124b04e5ed884
Part-of: odoo/odoo#94885