The case was NOT empty!
I have no idea what was in the case, but I do know there was something valuable in it.
How do I know this?
Well the case had gone through so many peoples hands in the movie that dont you think one of them might have opened it up to verify it was genuine?
I highly doubt gregor risked his life and limb on an empty case that he never even verified.
"What color is the boathouse at Hereford!?"
-Sam