I think now my problem is solved:

mysql> select * from t1 ;
+------+------+
| a    | b    |
+------+------+
|      | NULL |
+------+------+
1 row in set (0.00 sec)

mysql> select * from t1 where a is null ;
+------+------+
| a    | b    |
+------+------+
|      | NULL |
+------+------+
1 row in set (0.00 sec)
[download]

So, the value of column 'a' is not an empty string, but its 'NULL'!
Its very hard to see the difference :)