prober: fix WithBandwidthProbing behavior with optional tunAddress

1ed9bd76d6 meant to make tunAddress be optional.

Updates tailscale/corp#24635

Change-Id: Idc4a8540b294e480df5bd291967024c04df751c0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

prober/derp.go

@@ -51,7 +51,7 @@ type derpProber struct {
 	// Optional bandwidth probing.
 	bwInterval      time.Duration
 	bwProbeSize     int64
-	bwTUNIPv4Prefix *netip.Prefix
+	bwTUNIPv4Prefix *netip.Prefix // or nil to not use TUN
 
 	// Optionally restrict probes to a single regionCode.
 	regionCode string
@@ -78,17 +78,19 @@ type DERPOpt func(*derpProber)
 // `size` bytes will be regularly transferred through each DERP server, and each
 // pair of DERP servers in every region. If tunAddress is specified, probes will
 // use a TCP connection over a TUN device at this address in order to exercise
-// TCP-in-TCP in similar fashion to TCP over Tailscale via DERP
+// TCP-in-TCP in similar fashion to TCP over Tailscale via DERP.
 func WithBandwidthProbing(interval time.Duration, size int64, tunAddress string) DERPOpt {
 	return func(d *derpProber) {
 		d.bwInterval = interval
 		d.bwProbeSize = size
+		if tunAddress != "" {
 			prefix, err := netip.ParsePrefix(fmt.Sprintf("%s/30", tunAddress))
 			if err != nil {
 				log.Fatalf("failed to parse IP prefix from bw-tun-ipv4-addr: %v", err)
 			}
 			d.bwTUNIPv4Prefix = &prefix
 		}
+	}
 }
 
 // WithMeshProbing enables mesh probing. When enabled, a small message will be