title: Zf2 multicheckbox at least one element is always required
body: <p>I can't validate a zf2 form with multicheckbox because at least one checkbox is always required.
  I found a lot of reference to this issue (for example here - <a href="https://github.com/zendframework/zf2/issues/4845" rel="nofollow">https://github.com/zendframework/zf2/issues/4845</a>), but i didn't found a solution for this.
  Does anybody know how to solve this problem ?</p>
  
  <p>UPDATE: I use a doctrine 2 <a href="https://github.com/doctrine/DoctrineModule/blob/master/docs/form-element.md" rel="nofollow">objectmulticheckbox</a> which is extended from zf2 multichechbox. As is commented below the override of getInputFilterSpecification method, will solve the problem with form validation, but the values will still remain in database (values populated by objectmulticheckbox).</p>