[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