If you want your query to return empty columns, you have to add Nullable='TRUE' to the viewfields.

<FieldRef Name='Field1' Nullable='TRUE'/>

If you do not add the Nullable attribute, accessing the results of the query like this:

oListItemAvailable["Field1"] will give an Exception.

I’ve made posted a comment about this on the page in the MSDN http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spquery.viewfields.aspx