Use random node when there are no channels

cluster.go

@@ -1537,10 +1537,13 @@ func (c *ClusterClient) pubSub() *PubSub {
 				panic("node != nil")
 			}
 
-			slot := hashtag.Slot(channels[0])
-
 			var err error
+			if len(channels) > 0 {
+				slot := hashtag.Slot(channels[0])
 				node, err = c.slotMasterNode(slot)
+			} else {
+				node, err = c.nodes.Random()
+			}
 			if err != nil {
 				return nil, err
 			}

cluster_test.go

@@ -512,6 +512,14 @@ var _ = Describe("ClusterClient", func() {
 				return nil
 			}, 30*time.Second).ShouldNot(HaveOccurred())
 		})
+
+		It("supports PubSub.Ping without channels", func() {
+			pubsub := client.Subscribe()
+			defer pubsub.Close()
+
+			err := pubsub.Ping()
+			Expect(err).NotTo(HaveOccurred())
+		})
 	}
 
 	Describe("ClusterClient", func() {