create table test_null(v1 CHAR(5));
INSERT INTO test_null SELECT NULL FROM DUAL CONNECT BY LEVEL<=10;      
INSERT INTO test_null SELECT 'LVL'||LEVEL FROM DUAL CONNECT BY LEVEL<=10; 
UPDATE test_null
   SET V1 =''
WHERE V1 IS NULL;
COMMIT;
SELECT * FROM test_null WHERE V1 ='';   
SELECT * FROM test_null WHERE V1 IS NULL;

SQL> declare

  a char(5);
  b char(5);
  c varchar2(5);
  d varchar2(5);

begin

  a := '';
  b := null;
  c := '';
  d := null;

  dbms_output.put_line('char_empty_string $'||a||'$');
  dbms_output.put_line('char_null $'||b||'$');
  dbms_output.put_line('varchar_empty_string $'||c||'$');
  dbms_output.put_line('varchar_null $'||d||'$');

end;  2    3    4    5    6    7    8    9   10   11   12   13   14   15   16   17   18   19   20
 21  /
char_empty_string $     $
char_null $$
varchar_empty_string $$
varchar_null $$