Bad mistake by me: in my code i used Input::post('bla',null) and the problem here is (of course!) that if its not an unchecked checkbox, the input always exists but if the user did not type in anything, the value is an empty string and because the key inside $_POST exists, Input::post returns the empty string instead of null.
So all is correct with fuel, the problem sat in front of the screen :)
Thank you very much for your reply and sorry for this :(