UNION ALL query return wrong type, NULL value but get ''

[rongyousu]

the detail is that:
sql:
with a as
(
select 2 as id, '1234' as user_id
union all
select 1 as id, NULL as user_id
),
b as
(
select 4 as id, '543' as user_id
union all
select 3 as id, NULL as user_id
)
select id,user_id from a
union all
select id ,user_id from b

the result is :

my value is "NULL " ,but use with lable ,the result is issue, now is ''

[HappenLee]

Hi，It's look like is FE query plan's problem. I will try to fix the problem