Synchronization: local doc with null life cycle state should be updated

The following code in DefaultDocumentDifferencesPolicy can raise an NPE on local documents that have a null life cycle state. This can be the case if a "partial" synchronization happens. This should never happen but some cases were detected, most certainly due to Nuxeo 5.3.1 Jetty distribution being not transactional.

else if (modificationDate.getTimeInMillis() / 1000 != (long)tuple.getLastModification() / 1000
  || !doc.getCurrentLifeCycleState().equals(lifecycleState)) {
    tuple.setClientId(doc.getId());
    modifiedTuples.add(tuple);
}