A Null Script handle can be created in this manner:
_handle = 0 spawn {};
That is what any Script Handle becomes when a script is finished running, meaning it will test as true with:
scriptDone _handle;
and it returns "<NULL-script>" when converted to string. This in turn lets you initialize the variable with a completed Script Handle and lets you test the variable even though nothing else may have set the handle by creating a script with spawn or execVM. It also lets you store and manage script handles in arrays, and a few other sexy things.