nuke: don't run umount when no xargs args

nuke: don't run umount when no xargs args

Gets rid of this noise:

INFO:teuthology.nuke:Unmount any osd data directories...
INFO:teuthology.orchestra.run.err:Usage: umount -h | -V
INFO:teuthology.orchestra.run.err:       umount -a [-d] [-f] [-r] [-n] [-v] [-t vfstypes] [-O opts]
INFO:teuthology.orchestra.run.err:       umount [-d] [-f] [-r] [-n] [-v] special | node...
INFO:teuthology.orchestra.run.err:Usage: umount -h | -V
INFO:teuthology.orchestra.run.err:       umount -a [-d] [-f] [-r] [-n] [-v] [-t vfstypes] [-O opts]
INFO:teuthology.orchestra.run.err:       umount [-d] [-f] [-r] [-n] [-v] special | node...
...