[BUG] Predicate push down where sub query has distinct may throw NPE

[yangzhg]

Describe the bug
Predicate push down where sub query has distinct may throw NPE

To Reproduce
Steps to reproduce the behavior:

1. create table like

+--------------+--------------+------+-------+---------+---------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-------+---------+---------+
| event_day | DATETIME | No | true | NULL | |
| title | VARCHAR(600) | No | true | NULL | |
| report_value | VARCHAR(50) | No | false | NULL | REPLACE |
+--------------+--------------+------+-------+---------+---------+

2. exec query

```SELECT
 *
FROM
 (
 SELECT
 DISTINCT event_day,
 title
 FROM
 click_show_window
 ) a
WHERE
 a.title IS NOT NULL

4. See error

ERROR 1064 (HY000): errCode = 2, detailMessage = Unexpected exception: null

This is because DISTINCT generate grouping exprs in agginfo, but this clause does not have a group by clause

[HangyuanLiu]

The bug is same as #4263 , I have fix it at #4294