gluster --xml: survive missing fields

Do not fail gluster --xml volume status detail with an empty output
if some field is missing.

Required for NetBSD to pass tests/bugs/bug-861542.t

BUG: 1129939
Change-Id: I7e2097dee5e18a5f3bb8fe6f14be53b33f4aa6b1
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9024
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>