Even if I allow null values and replace all the NV instances with ""(nothing), I still get zero in that cell.

I suspect that this isn't doing what you think it is. Where MySql is concerened, "" is not the same as NULL.

Consider the following example:

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)
[download]

If you want to represent your NV values with NULL, then you need to explicity specify NULL when you insert. (If you are using placeholders, then I think an undef value will evaluate to NULL, but you'd have to check that).

Update: Actually (and before somebody jumps on me), that statement above isn't quite true. eg:

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

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

Of course, because NULL is the default - if you just insert without giving anything at all - you'll get NULL :)

Hope this helps,
Darren :)