release/vm: finish fixing WITHOUT_QEMU

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: e6e5cd297ab4 ("release/vm: fix setting NO_ROOT and WITHOUT_QEMU")
Sponsored by: DARPA, AFRL

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D49666