When running `tailscale exit-node list`, an empty city or country name
should be displayed as a hyphen "-". However, this only happened when
there was no location at all. If a node provides a Hostinfo.Location,
then the list would display exactly what was provided.
This patch changes the listing so that empty cities and countries will
either render the provided name or "-".
Fixes#16500
Signed-off-by: Simon Law <sfllaw@tailscale.com>