dan@oraspb>; create table test (test_id number( 9 ))
   2   /

Table created.

dan@oraspb>; create table test2 (test_id number( 9 ))
   2   /

Table created.

dan@oraspb>; insert into test(test_id) values ( 1 );

 1  row created.

dan@oraspb>; insert into test(test_id) values ( 2 );

 1  row created.

dan@oraspb>; 
dan@oraspb>; insert into test(test_id) values ( 1 );

 1  row created.

dan@oraspb>; insert into test(test_id) values (null);

 1  row created.

dan@oraspb>; 
dan@oraspb>; commit;

Commit complete.

dan@oraspb>;                  
dan@oraspb>; set null NULL;
dan@oraspb>; select * from test where test_id not in (select test_id from test2);

   TEST_ID
 ----------
 
          1 
          2 
          1 
NULL

dan@oraspb>; select * from test where test_id not in (select null from dual);

no rows selected