Issue #35: Don't cache empty data.

Caching paths in Zookeeper where 'data' and 'children' lists are
entirely empty doesn't make sense. In the event that we get an update
to a Watcher where 'children'/'data' are empty, the most likely scenario
is that we were watching a path with Ephemeral nodes that were
providing a service (ie, your database server at /prod/master_db) and
for some reason that server has become un-registered from Zookeeper. If
there are no other nodes in the children list, then its highly likely
you do not want to cache this "empty" result list.