...is it al right to specify an ISNULL function in the select list, for example:
       SELECT Col1, ISNULL(Col2, 0) FROM Tab1
       FOR XML AUTO, ELEMENTS
    or
    SELECT Col1, ISNULL(Col2, '') FROM Tab1
       FOR XML AUTO, ELEMENTS
   
    ...the result for Col2 will look like <Col2>0</Col2> or <Col2></Col2>