cmd/containerboot: ensure that subnet routes can be unset. (#10734)

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>

cmd/containerboot/main_test.go

@@ -218,6 +218,28 @@ func TestContainerBoot(t *testing.T) {
 				},
 			},
 		},
+		{
+			Name: "empty routes",
+			Env: map[string]string{
+				"TS_AUTHKEY": "tskey-key",
+				"TS_ROUTES":  "",
+			},
+			Phases: []phase{
+				{
+					WantCmds: []string{
+						"/usr/bin/tailscaled --socket=/tmp/tailscaled.sock --state=mem: --statedir=/tmp --tun=userspace-networking",
+						"/usr/bin/tailscale --socket=/tmp/tailscaled.sock up --accept-dns=false --authkey=tskey-key --advertise-routes=",
+					},
+				},
+				{
+					Notify: runningNotify,
+					WantFiles: map[string]string{
+						"proc/sys/net/ipv4/ip_forward":          "0",
+						"proc/sys/net/ipv6/conf/all/forwarding": "0",
+					},
+				},
+			},
+		},
 		{
 			Name: "routes_kernel_ipv4",
 			Env: map[string]string{