R600/SI: return undef instead of null for skipped arguments

This is a candidate for the stable branch.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=64694

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>