Hi @fabian
Please keep in mind this is a community forum, not a support desk. If you wish for one on one support, please email us here: http://support.advancedcustomfields.com/
As for you question, the radio button is not a type of input which can allow a null value. It was a mistake for it to ever allow such behavior.
The acf/validate_value filter can be used on any value to return a customized validation result which is performed on submission of the post and displayed via AJAX.
Does that help?
