[FIX] website_slides: order channels by the natural order when unspecified

In the backend, the user has the possibility in the channels list to drag and drop
channels to define their order, thanks to the `sequence`.

This order was then not respected in the front-end when displaying the slides channel
`/slides/all`.
This was basically not possible for the user to control the order in which
the channels were displayed.

With this revision, when the order is not specified to the controller,
or is wrong, it fallbacks to `None`, which therefore follows the slides channel natural
order, which is `sequence, id`. The user can now control the order of the slides channels
in the front-end.

closes odoo/odoo#33745

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