searching for non-empty fields

[Brian Whitman]

I have a large index with a field for a URL. For some reason or  
another, sometimes a doc will get indexed with that field blank. This  
is fine but I want a query to return only the set URL fields...

If I do a query like:
q=URL:[* TO *]

I get a lot of empty fields back, like:

<doc><str name="URL"/></doc>
<doc><str name="URL"/></doc>
<doc><str name="URL">http://thing.com</str></doc>

What I can query for to remove the empty fields?