To support common shell patterns (test -n and -z) we want WITHOUT_QEMU
to be set and not empty in mk-vmimage.sh if it is defined at all in the
Makefile.
Fix a case I missed previously.
Reported by: cperciva
Fixes: f3a73a7055a8 ("release/vm: fix setting NO_ROOT and WITHOUT_QEMU")
Sponsored by: DARPA, AFRL