glusterfs.list_volumes should return the expected type

We are expecting a list, not a None.

     Comment: An exception occurred in this state: Traceback (most recent call last):
                File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1517, in call
                  **cdata['kwargs'])
                File "/usr/lib/python2.7/dist-packages/salt/states/glusterfs.py", line 178, in started
                  if name not in volumes:
              TypeError: argument of type 'NoneType' is not iterable