Test case:

recreate table bugtest (id int)
insert into bugtest (id) values (123)

recreate procedure p_bugtest (in_id int)
  returns (cnt int)
as
  declare predicate varchar(1000);
begin
  if (:in_id is null) then
    predicate = ' ? is null';
  else
    predicate = ' id = ?';

  execute statement ('select count(*) from bugtest where' || predicate) (:in_id)
  into :cnt;
end

execute procedure p_bugtest (123)
-- cnt = 1
execute procedure p_bugtest (null)
-- cnt = 1
execute procedure p_bugtest (123)
-- cnt = 0 !!!