Commit 48c043d0 "hmp: human-monitor-command: stop using the Memory
chardev driver" left us "if string is non-empty, duplicate it, else
duplicate the empty string".  Meh.  Duplicate it unconditionally.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-2-armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>