Submitted by: @asfernandes

Both cases returns "0, 0" instead of "null, null".

create table t1 (n1 integer, n2 integer);

-- Case 1:
merge into t1
using (
select 1 x
from rdb$database
where 1 = 0
) on 1 = 1
when not matched then
insert values (1, 11)
returning n1, n2;

-- Case 2:
merge into t1
using (
select 1 x
from rdb$database
where 1 = 1
) on 1 = 0
when not matched and 1 = 0 then
insert values (1, 11)
returning n1, n2;

Commits: 551dbdb FirebirdSQL/fbt-repository@7b257ba