lsobj: Do not print prefix if doesn't exist.
We are printing the prefix in lsobj even if it doesn't exist or have objects under it [0]. This is inconsistent with the expected behavior.
swirl:~/nm-dns> lsobj droot:foo
foo/
swirl:~/nm-dns> lsobj droot:this-does-not-exist
this-does-not-exist/
swirl:~/nm-dns> lsobj droot:nor-does-this/
nor-does-this/
[0] https://gitlab.umiacs.umd.edu/staff/umobj/blob/master/bin/lsobj#L29