handle query_event COMMIT

#1059 中出现了 query_event query="COMMIT" 的情况

```
[WARN] reader: mysql.reader: QueryEvent is not recognized. will still execute: job=dtle-track query=COMMIT gno=89248399
```

原场景(RDS 5.6)未能复现.

但发现如下case可触发

```
set gtid_next = 'some_uuid:some_gno';
begin;
commit;
-- binlog中形成query_event BEGIN 和 COMMIT的空事务
```

需验证dtle能否处理此场景及后续复制.

已知在`gtid_next='automatic'`时, 空事务不会写入binlog.