[FIX] website_event_track_exhibitor: fix sponsor 'midnight' opening hours

Currently, configuring opening hours for event sponsors 'at midnight' is not
handled properly.

If you set either the start time or the end time opening hours to "00:00",
the sponsor will be considered as always open.

We fix that by correctly taking into account that configuration.

(Note: if the user configures a sponsor to be open 'until midnight', we assume
it means midnight 'the next day').

Tests have been altered to take this into account.

Task-3459777

closes odoo/odoo#131348

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>