I am not entirely sure whether this is a usage problem by myself but using
CHtml::listData(Model::model()->findAll(array('order' => 'full_name')), 'id', 'display')
to fill in the values array there is no "void" option in the generated chosen-widget, i.e. some option is always selected on submit, even if the chosen widget was never manipulated.
The Chosen documentation clearly states the option for a way to deselect any selected value: When a single select box isn't a required field, you can set allow_single_deselect: true and Chosen will add a UI element for option deselection. This will only work if the first option has blank text.
My guess would be that no blank first option is added automatically?