[FIX] website_slides_survey: fix certificate preview

before this commit,
it was raising an error while try to preview the certificate

after this commit,
prevent the preview of certificate, as it is not possible to
set the preview of certificate

task- 2623276

closes odoo/odoo#75116

Signed-off-by: awa-odoo <awa-odoo@users.noreply.github.com>