I wonder.... ?
I think(?) EMP is an atmospheric effect, i.e. the nuke's radiation super heats the atmosphere that then produces this huge plasma from atmospheric gas that in turn throws out an enormous magnetic field transient. If a nuke was popped off in a vacuum then it might not produce much of an EMP. Of course, EMP is something that you might want to test for if you're developing a weapon.
As an aside, years ago I heard that the Pioneer Venus orbiter (Pioneer 12) saw a large flash in the Venusian atmosphere. At the time, people were quietly speculating whether this was a Soviet nuclear weapons test (it almost certainly wasn't).
Gary