Incorrect predicate pushdown for groupby with grouping sets

Consider the following query:

CREATE TABLE T1(a STRING, b STRING, s BIGINT);
INSERT OVERWRITE TABLE T1 VALUES ('aaaa', 'bbbb', 123456);

SELECT * FROM (
SELECT a, b, sum(s)
FROM T1
GROUP BY a, b GROUPING SETS ((), (a), (b), (a, b))
) t WHERE a IS NOT NULL;

When hive.optimize.ppd is enabled (and hive.cbo.enable=false), the query will output:

NULL	NULL	123456
NULL	bbbb	123456
aaaa	NULL	123456
aaaa	bbbb	123456

We can see the predicate "a IS NOT NULL" takes no effect, which is incorrect.

When performing PPD optimization for a GBY operator, we should make sure all grouping sets contains the processing expr before pushdown. otherwise the expr value after GBY is changed and the result is wrong.