XML : pass an empty argument into an scheduled task xml file

I am trying to run a batch file when some specific events occurs and I want to pass some null arguments to one of my tasks. The problem is that I don't know how. I have tried a lot of combinations but without success. My xml files looks something like:

      ........          <ValueQueries>              <Valuename="TargetUserName">Event/EventData/Data[@Name='TargetUserName']</Value>              <Value name="EventID">Event/System/EventID</Value>        </ValueQueries>        ......       <Exec>        <Command>C:\passwd.bat</Command>        <Arguments>$(TargetUserName) $(SubjectUserName) $(EventID) </Arguments>      </Exec>    

Now, I would like to pass an empty argument [between $(TargetUserName) and $(EventID)] for one of my events but I didn't manage to do this yet.

Any suggestions ?