[FIX] hr_holidays: fix no id to export when selecting all in tree view

Reproduce the issue

    - Install Time Off
    - Select all the lines
    - Action > Export

    The file has only columns but no data

Cause

    In `hr_leave_type.py` the `_search` method's limit is `False` when
    we select all the lines.

    In 04e85efb, we return the leaves ids with `[:limit]` so `[:False]`
    that always returns an empty array instead of `[:None]` that returns
    the full list.

This commit replaces the `[:limit]` by `[:limit or None]`

OPW-2126109

closes odoo/odoo#40539

Signed-off-by: Jason Van Malder <jasonvanmalder@users.noreply.github.com>