bug: None can be returned when executing a watch on a directory with no nodes.

[bendemott]

solr-zkutil watch /overseer_elect --env PILOT

ERROR:kazoo.recipe.watchers:'NoneType' object has no attribute 'decode'
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\kazoo\recipe\watchers.py", line 161, in _log_func_exception
    result = self._func(data, stat, event)
  File "C:\Python27\lib\site-packages\solrzkutil\__init__.py", line 619, in watch_data
    data = data.decode('utf-8')
AttributeError: 'NoneType' object has no attribute 'decode'
Traceback (most recent call last):
  File "C:\Python27\Scripts\solr-zkutil", line 13, in <module>
    sys.exit(main(sys.argv))
  File "C:\Python27\lib\site-packages\solrzkutil\__init__.py", line 1002, in main
    watch(zookeepers=args['zookeepers'], node=args['node'], leader=args['leader'])
  File "C:\Python27\lib\site-packages\solrzkutil\__init__.py", line 614, in watch
    @zk.DataWatch(node)
  File "C:\Python27\lib\site-packages\kazoo\recipe\watchers.py", line 151, in __call__
    self._get_data()
  File "C:\Python27\lib\site-packages\kazoo\recipe\watchers.py", line 36, in wrapper
    return func(*args, **kwargs)
  File "C:\Python27\lib\site-packages\kazoo\recipe\watchers.py", line 205, in _get_data
    self._log_func_exception(data, stat, event)
  File "C:\Python27\lib\site-packages\kazoo\recipe\watchers.py", line 161, in _log_func_exception
    result = self._func(data, stat, event)
  File "C:\Python27\lib\site-packages\solrzkutil\__init__.py", line 619, in watch_data
    data = data.decode('utf-8')
AttributeError: 'NoneType' object has no attribute 'decode'