sink/mq: fix empty value in open protocol

cdc/sink/codec/json.go

@@ -395,6 +395,12 @@ func (d *JSONEventBatchEncoder) EncodeCheckpointEvent(ts uint64) (*MQMessage, er
 
 // AppendRowChangedEvent implements the EventBatchEncoder interface
 func (d *JSONEventBatchEncoder) AppendRowChangedEvent(e *model.RowChangedEvent) (EncoderResult, error) {
+	// Some transactions could generate empty row change event, such as
+	// begin; insert into t (id) values (1); delete from t where id=1; commit;
+	// Just ignore these row changed events
+	if len(e.Columns) == 0 && len(e.PreColumns) == 0 {
+		return EncoderNoOperation, nil
+	}
 	keyMsg, valueMsg := rowEventToMqMessage(e)
 	key, err := keyMsg.Encode()
 	if err != nil {

cdc/sink/codec/json_test.go

@@ -304,6 +304,28 @@ func (s *batchSuite) TestMaxBatchSize(c *check.C) {
 	c.Check(sum, check.Equals, 10000)
 }
 
+func (s *batchSuite) TestEmptyMessage(c *check.C) {
+	defer testleak.AfterTest(c)()
+	encoder := NewJSONEventBatchEncoder()
+	err := encoder.SetParams(map[string]string{"max-batch-size": "64"})
+	c.Check(err, check.IsNil)
+
+	emptyEvent := &model.RowChangedEvent{
+		CommitTs: 1,
+		Table:    &model.TableName{Schema: "a", Table: "b"},
+		Columns:  []*model.Column{},
+	}
+
+	for i := 0; i < 10000; i++ {
+		r, err := encoder.AppendRowChangedEvent(emptyEvent)
+		c.Check(r, check.Equals, EncoderNoOperation)
+		c.Check(err, check.IsNil)
+	}
+
+	messages := encoder.Build()
+	c.Assert(messages, check.HasLen, 0)
+}
+
 func (s *batchSuite) TestDefaultEventBatchCodec(c *check.C) {
 	defer testleak.AfterTest(c)()
 	s.testBatchCodec(c, func() EventBatchEncoder {