Make sure the NotEmpty validator is not already in chain
Merge branch 'InputFilter-NotEmpty' of git://github.com/grizzm0/zf2 into hotfix/3683
[#3683] Added unit test for behavior

- Allows NotEmpty validator anywhere in chain (not just as first
  validator in chain)
Merge branch 'hotfix/3683'

Close #3683
Fixes #3678