And I get, no matter how I look at it, an empty string for $@ that tests as true

The trueness just goes to prove that $@ is not an empty string ... and that further proves that you're not looking at (or at least seeing) $@ in the right way.

Of course, that doesn't answer your question, but maybe it highlights what you need to focus on.
Good luck with it - an intriguing situation :-)

Cheers,
Rob