vmon: suppress "(null)" name in snapshot filenames

When this code changed to use a local, potentially heap-allocated
name variable, it started producing "(null)" when no -n/--name
was supplied, that wasn't intended.

Just use a "" name when NULL, enabling bare date-derived snapshot
filenames.  This seems preferable since even if you supplied an
empty -n/--name you'd get a hyphen at the start of the name.  I
can see scenarios where you have unnamed files labeled by the
output dir instead.

0 files changed, 0 insertions, 0 deletions