A traceback was shown when 'None' background was chosen to configure the onboarding panel's background image (css class). Supposing no one built anything with a `o_onboarding_False` class, we also clean the result for a `False` value while we are here. Task-3104723 closes odoo/odoo#108801 X-original-commit: 8a8da706e55073c48aa3cdc21201b4552230de87 Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>