Union queries with group by clause failure

### Affected Version
0.21.1

### Description
This query is working in version 0.19.0
SELECT count(*) as cnt
0 as a
0 as b
0 as c
FROM "datasource"
WHERE FLOOR(__time TO HOUR) >= '2021-08-15 00:00:00' AND FLOOR(__time TO HOUR) <= '2021-08-15 23:00:00'
GROUP BY item_id
UNION ALL
SELECT 0 as cnt
sum(a) as a
sum(b) as b
sum(c) as c
FROM "datasource"
WHERE FLOOR(__time TO HOUR) >= '2021-08-15 00:00:00' AND FLOOR(__time TO HOUR) <= '2021-08-15 23:00:00'

but in later version we're getting the following error:
Error: Unknown exception
Error while applying rule DruidQueryRule(AGGREGATE_PROJECT), args [rel#170209:LogicalProject.NONE.[

Tested in various clusters from 1 data node up to 6 data nodes

Thanks

Start by reproducing the supplied UNION ALL query against Apache Druid 0.21.1 and comparing it with 0.19.0. Trace the reported DruidQueryRule(AGGREGATE_PROJECT) failure to identify the regression; done means the query no longer fails and its prior behavior is covered by a regression check.

Written by the indexing model from the issue text.