Marker class used with annotations to indicate "no class". This is
a silly but necessary work-around -- annotations cannot take nulls
as either default or explicit values. Hence for class values we must
explicitly use a bogus placeholder to denote equivalent of
"no class" (for which 'null' is usually the natural choice).
Note that since 2.4, most (but not all!
a notable exception}) usage should start using
Void instead as the "not defined" marker.