Fix dictionary lookup on NoneType in jetty options

When the jetty: section is mentioned, but does not contain active
configuration items self._conf['m2ee'].get('jetty', {}) returns None.
The in operator doesn't like to operate on a NoneType.

    jetty:
     # comment
     #key: value

This bug was introduced in commit 682259dbefe80bad5f23555b140a95c3e2f829d0