Ensure that JSON.GET returns Nil response

Updated JSONCmd.readReply to return redis.Nil if no results
Added a doc line for Val() and Expanded() methods of JSONCmd
Added a test case for non existent keys in json_test.go

json.go

@@ -82,6 +82,7 @@ func (cmd *JSONCmd) SetVal(val string) {
 	cmd.val = val
 }
 
+// Val returns the result of the JSON.GET command as a string.
 func (cmd *JSONCmd) Val() string {
 	if len(cmd.val) == 0 && cmd.expanded != nil {
 		val, err := json.Marshal(cmd.expanded)
@@ -100,6 +101,7 @@ func (cmd *JSONCmd) Result() (string, error) {
 	return cmd.Val(), cmd.Err()
 }
 
+// Expanded returns the result of the JSON.GET command as unmarshalled JSON.
 func (cmd JSONCmd) Expanded() (interface{}, error) {
 	if len(cmd.val) != 0 && cmd.expanded == nil {
 		err := json.Unmarshal([]byte(cmd.val), &cmd.expanded)
@@ -112,10 +114,10 @@ func (cmd JSONCmd) Expanded() (interface{}, error) {
 }
 
 func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
-	// nil response from JSON.(M)GET (cmd.baseCmd.err will be "redis: nil")
-	if cmd.baseCmd.Err() == Nil {
+
+	if cmd.baseCmd.Err() != nil {
 		cmd.val = ""
-		return Nil
+		return cmd.baseCmd.Err()
 	}
 
 	if readType, err := rd.PeekReplyType(); err != nil {
@@ -126,6 +128,9 @@ func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 		if err != nil {
 			return err
 		}
+		if size == 0 {
+			return Nil
+		}
 
 		expanded := make([]interface{}, size)
 
@@ -141,6 +146,7 @@ func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 			return err
 		} else if str == "" || err == Nil {
 			cmd.val = ""
+			return Nil
 		} else {
 			cmd.val = str
 		}

json_test.go

@@ -123,9 +123,14 @@ var _ = Describe("JSON Commands", Label("json"), func() {
 			Expect(err).NotTo(HaveOccurred())
 			Expect(resGet).To(Equal("[[10,20,30,40],[5,10,20,30]]"))
 
+			_, err = client.JSONGet(ctx, "this-key-does-not-exist", "$").Result()
+			Expect(err).To(HaveOccurred())
+			Expect(err).To(BeIdenticalTo(redis.Nil))
+
 			resArr, err := client.JSONArrIndex(ctx, "doc1", "$.store.book[?(@.price<10)].size", 20).Result()
 			Expect(err).NotTo(HaveOccurred())
 			Expect(resArr).To(Equal([]int64{1, 2}))
+
 		})
 
 		It("should JSONArrInsert", Label("json.arrinsert", "json"), func() {