[FIX] hr_holidays: Allow overlapping if not employee

Issue:

  When creating a leave there is an error when you use by "employee tag"
  but it works when you use "by employee".

Solution:

  cherry-pick of:
  https://github.com/odoo/odoo/commit/394eedc1e4b6b9871b360d6ab168005c2731d3ff
  https://github.com/odoo/odoo/commit/c89c0e70da1e09c43eee8e76c7094dcc114c451b



opw-2525086

closes odoo/odoo#74170

Signed-off-by: bon-odoo <nboulif@users.noreply.github.com>