events with pricesets having non-required options won't submit

create a price set containing radio options. do not make it required.
create an event and use that price set

if the user selects "none" as the option, the form will not allow you to advance, returning:
"Select at least one option from Event Fee(s)."

if no price set field is required, we should allow the participant to continue without selecting a fee option.

see this example:
http://sandbox.drupal.civicrm.org/civicrm/event/register?reset=1&id=7