Query for Multiple Rows as XML 'NULL' question

There are two fields in the database.

Foo = null (real null)

Foo1 = '' (blank string)

In my return xml the nodes are as follows:

<Foo>null</Foo>

<Foo1/>

Why is it that if there is a null in the database, that it puts the string null, and then when there is a blank string, the node is empty?

How can I get the null value to act like the blank value?

Cheers