let XReadGroup skip empty message and process next message (redis#1243)

* let XReadGroup skip empty message and process next message

command.go

@@ -1004,10 +1004,14 @@ func xMessageSliceParser(rd *proto.Reader, n int64) (interface{}, error) {
 			}
 
 			v, err := rd.ReadArrayReply(stringInterfaceMapParser)
-			if err != nil {
+			if err != nil && err != proto.Nil {
 				return nil, err
 			}
 
+			if v == nil || err == proto.Nil {
+				v = make(map[string]interface{})
+			}
+
 			msgs[i] = XMessage{
 				ID:     id,
 				Values: v.(map[string]interface{}),

commands_test.go

@@ -3623,6 +3623,27 @@ var _ = Describe("Commands", func() {
 				Expect(n).To(Equal(int64(1)))
 			})
 
+			It("should XReadGroup skip empty", func() {
+				n, err := client.XDel("stream", "2-0").Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(n).To(Equal(int64(1)))
+
+				res, err := client.XReadGroup(&redis.XReadGroupArgs{
+					Group:    "group",
+					Consumer: "consumer",
+					Streams:  []string{"stream", "0"},
+				}).Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(res).To(Equal([]redis.XStream{{
+					Stream: "stream",
+					Messages: []redis.XMessage{
+						{ID: "1-0", Values: map[string]interface{}{"uno": "un"}},
+						{ID: "2-0", Values: map[string]interface{}{}},
+						{ID: "3-0", Values: map[string]interface{}{"tres": "troix"}},
+					}},
+				}))
+			})
+
 			It("should XGroupCreateMkStream", func() {
 				err := client.XGroupCreateMkStream("stream2", "group", "0").Err()
 				Expect(err).NotTo(HaveOccurred())