[FIX] web_editor: properly start website edit mode as restricted editor

It was impossible to open the website's edit mode as a user with
"restricted editor" permissions. The reason was that wysiwyg_multizone
was trying to manipulate data that is not injected into the html node
when `editable` is false (see `website.layout` template).

closes odoo/odoo#31702

Signed-off-by: Pierre Paridans <pparidans@users.noreply.github.com>