You are most likely pushing a string 'NULL' to the table, rather then an actual NULL, but other things may be going on as well, an illustration:
mysql> CREATE TABLE date_test (pdd DATE NOT NULL);
Query OK, 0 rows affected (0.11 sec)
mysql> INSERT INTO date_test VALUES (NULL);
ERROR 1048 (23000): Column 'pdd' cannot be null
mysql> INSERT INTO date_test VALUES ('NULL');
Query OK, 1 row affected, 1 warning (0.05 sec)
mysql> show warnings;
+---------+------+------------------------------------------+
| Level | Code | Message |
+---------+------+------------------------------------------+
| Warning | 1265 | Data truncated for column 'pdd' at row 1 |
+---------+------+------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT * FROM date_test;
+------------+
| pdd |
+------------+
| 0000-00-00 |
+------------+
1 row in set (0.00 sec)
mysql> ALTER TABLE date_test MODIFY COLUMN pdd DATE NULL;
Query OK, 1 row affected (0.15 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> INSERT INTO date_test VALUES (NULL);
Query OK, 1 row affected (0.06 sec)
mysql> SELECT * FROM date_test;
+------------+
| pdd |
+------------+
| 0000-00-00 |
| NULL |
+------------+
2 rows in set (0.00 sec)
