[FIX] bus: getItemFromStorage error when no dbuuid is provided

The `bus_service` uses the `dbuuid` key from session in order to
know when to drop the last notification id from the local storage:
when the db changes.

The issue is that public pages don't fetch the session, this results
in undefined being added into the local storage. The multi tab service
speculates on the fact that the value stored will always be json
parsable.

This commit fixes the issue by taking into account the fact that session
dbuuid can be undefined. Moreover, the multi tab service has been
updated to return the raw value if it is not parsable.

closes odoo/odoo#100652

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>