cannot create empty file

I'm unable to figure out how to create an empty file.
If I remove <warning_file></warning_file> , then I get a file with only the warning.
Otherwise, it fails with an error.  Is this possible?

<config>
  <file>
    <warning_file></warning_file>
    <allow_empty/>
    <source>
      <plain>somezerobytefile</plain>
    </source>
  </file>
</config>