A Tailnet node can be told to stop advertise subnets by passing
an empty string to --advertise-routes flag.
Respect an explicitly passed empty value to TS_ROUTES env var
so that users have a way to stop containerboot acting as a subnet
router without recreating it.
Distinguish between TS_ROUTES being unset and empty.
Updates tailscale/tailscale#10708
Signed-off-by: Irbe Krumina <irbe@tailscale.com>