Transact-SQL And XML Help Needed.

Ok here is the situation I need a little help with.

I am passing an XML document to a stored procedure. My XML document looks like this:
<ROOT>
<customer ID="10" Address="1234 Somewhere"></customer>
<customer ID="20" Address=""></customer>
</ROOT>

Now I use sp_xml_preparedocument and OpenXML to Insert the XML ID and Address attributes into a table in my database. The table has 2 fields (ID int) and (Address varchar(100) nullable)

The result of the insertion is 2 rows in my table with the ID field containing (10 and 20) and the Address field containing (1234 Somewhere and an empty string).

My first question is, why isn't the second row Address field in my database null instead of an empty string since thats what I would like happen. Is it because the XML Address atribute = "".

If that is the case then that should bring up my second question. Since the OpenXML inserts Address="" as an empty string instead of NULL, why doesn't a regular TransactSQL INSERT statement called from ASP does the same thing?? If I lost you here then let me give a quick example.
An html web form with a text field (txtAddress). The user leaves the text box empty and submits the form. My ASP takes the value of the form, Request.Form("txtAddress") and passes it to stored proceudre through an ASP parameter object. My stored procedure simply does an INSERT into tablename (ID, Address) VALUES (@passedID, @PassedAddress). Now the outcome in the database is that the Address field is NULL and not an empty string.

That is why im confused. Why arent the OpenXML and the straight-forward insertion yield the same result (either null or empty string). It seems to me that an empty text box is an empty string and not null. ASP's ISNull(txtAddress) agrees and returns false.

Any insight ot help on this issue is greatly appreciated.