Bug EXPLAIN and UNION

Environment: PostgreSQL 14.8 on Debian, Citus 11.0.4, 1-node cluster
Code:
```postgres
CREATE TABLE dist_table(id int primary key);
SELECT create_distributed_table('dist_table','id',shard_count:=2);

-- Ok. Return empty table
SELECT id FROM dist_table
UNION
SELECT id FROM dist_table
;
-- Get plan with error
EXPLAIN
SELECT id FROM dist_table
UNION
SELECT id FROM dist_table
;
```
```
Custom Scan (Citus Adaptive) (cost=0.00..0.00 rows=0 width=0)
-> Distributed Subplan 34_1
-> Custom Scan (Citus Adaptive) (cost=0.00..0.00 rows=100000 width=4)
Task Count: 2
Tasks Shown: All
-> Task
Node: host=127.0.0.1 port=54331 dbname=citus_test
-> Seq Scan on dist_table_102008 dist_table (cost=0.00..35.50 rows=2550 width=4)
-> Task
Node: host=127.0.0.1 port=54331 dbname=citus_test
-> Seq Scan on dist_table_102009 dist_table (cost=0.00..35.50 rows=2550 width=4)
-> Distributed Subplan 34_2
-> Custom Scan (Citus Adaptive) (cost=0.00..0.00 rows=100000 width=4)
Task Count: 2
Tasks Shown: All
-> Task
Node: host=127.0.0.1 port=54331 dbname=citus_test
-> Seq Scan on dist_table_102008 dist_table (cost=0.00..35.50 rows=2550 width=4)
-> Task
Node: host=127.0.0.1 port=54331 dbname=citus_test
-> Seq Scan on dist_table_102009 dist_table (cost=0.00..35.50 rows=2550 width=4)
Task Count: 1
Tasks Shown: All
-> Task
Error: Could not get remote plan.
```