We don't expose an allow blank option for unique field validation. You can hand code the update pretty easily by changing line 13 from:
$WAFV_Errors .= WAValidateUnique("RTMFi",$RTMFi,$database_RTMFi,"registered_apps","acc_id","none,none,NULL","0","acc_username","',none,''","".(((isset($_POST["username"]))?$_POST["username"]:"")) ."",true,2);
to:
if (!empty($_POST["username"])) $WAFV_Errors .= WAValidateUnique("RTMFi",$RTMFi,$database_RTMFi,"registered_apps","acc_id","none,none,NULL","0","acc_username","',none,''","".(((isset($_POST["username"]))?$_POST["username"]:"")) ."",true,2);