Support `is [not] null` for where_predicate

[asiroliu]

验证：
version:

9.9.9.9-master-05c9de5

1. create dtle job, set where is c_int is not null
2. src prepare data

create table `tb1`(id int(11) not null primary key auto_increment, c_int int)ENGINE=InnoDB DEFAULT CHARSET=utf8;
insert into `test`.`tb1` values (null,-2147483648);
insert into `test`.`tb1` values (null,NULL);

3. check results

# src mysql
mysql> select * from tb1;
+----+-------------+
| id | c_int       |
+----+-------------+
|  1 | -2147483648 |
|  2 |        NULL |
+----+-------------+
2 rows in set (0.00 sec)

# dest mysql
mysql> select * from tb1;
+----+-------------+
| id | c_int       |
+----+-------------+
|  1 | -2147483648 |
+----+-------------+
1 row in set (0.00 sec)