[FIX] website: prevent traceback in pages kanban view

Since [this other commit], a traceback is displayed when the user wants
to see the pages of his site when he has only one site. Steps to
reproduce the bug fixed by this commit:
- Launch Odoo without any app installed and without demo data
- Connect to the DB
- Activate the Website module
- Skip the configurator
- Choose a theme
- Discard edit mode
- Go to Site > Pages
- Go to kanban view

=> An error is displayed because the record has no website_id.

[this other commit]: https://github.com/odoo/odoo/commit/f23399db1dfb816a68ecf1be4367234b6e9a28bc



task-3316267

closes odoo/odoo#121016

Signed-off-by: Romain Derie (rde) <rde@odoo.com>