Fix empty host string when a cluster launched with one single instance

If a cluster is launched on only 1 redis instance, the result of `cluster slots` command may look like

    127.0.0.1:7000> cluster slots
    1) 1) (integer) 0
       2) (integer) 16383
       3) 1) ""
          2) (integer) 7000

The host property is set as an empty string, and this would finally make `socket.getaddrinfo` fail.