mysql> create table foo (Year year(2) default null);
Query OK, 0 rows affected (0.06 sec)
mysql> insert into foo values ("");
Query OK, 1 row affected (0.00 sec)

mysql> select * from foo;
+------+
| Year |
+------+
|   00 |
+------+
1 row in set (0.00 sec)

mysql> insert into foo values (NULL);
Query OK, 1 row affected (0.01 sec)

mysql> select * from foo;
+------+
| Year |
+------+
|   00 |
| NULL |
+------+
2 rows in set (0.01 sec)

##</code><code>##

mysql> insert into foo values ();
Query OK, 1 row affected (0.00 sec)

mysql> select * from foo;
+------+
| Year |
+------+
|   00 |
| NULL |
| NULL |
+------+