pkg/ipnet: Teach IPNet.String() to handle nil receivers

Instead of panicking.  This makes some comparisons easier, as you can
see from the test changes.

pkg/ipnet/ipnet.go

@@ -16,6 +16,15 @@ type IPNet struct {
 	net.IPNet
 }
 
+// String returns a CIDR serialization of the subnet, or an empty
+// string if the subnet is nil.
+func (ipnet *IPNet) String() string {
+	if ipnet == nil {
+		return ""
+	}
+	return ipnet.IPNet.String()
+}
+
 // MarshalJSON interface for an IPNet
 func (ipnet IPNet) MarshalJSON() (data []byte, err error) {
 	if reflect.DeepEqual(ipnet.IPNet, emptyIPNet) {

pkg/ipnet/ipnet_test.go

@@ -38,23 +38,19 @@ func TestUnmarshal(t *testing.T) {
 			Mask: net.IPv4Mask(255, 255, 255, 0),
 		}},
 	} {
-		data, err := json.Marshal(ipNetIn)
-		if err != nil {
-			t.Fatal(err)
-		}
+		t.Run(ipNetIn.String(), func(t *testing.T) {
+			data, err := json.Marshal(ipNetIn)
+			if err != nil {
+				t.Fatal(err)
+			}
 
-		t.Run(string(data), func(t *testing.T) {
 			var ipNetOut *IPNet
-			err := json.Unmarshal(data, &ipNetOut)
+			err = json.Unmarshal(data, &ipNetOut)
 			if err != nil {
 				t.Fatal(err)
 			}
 
-			if ipNetIn == nil {
-				if ipNetOut != nil {
-					t.Fatalf("%v != %v", ipNetOut, ipNetIn)
-				}
-			} else if ipNetOut.String() != ipNetIn.String() {
+			if ipNetOut.String() != ipNetIn.String() {
 				t.Fatalf("%v != %v", ipNetOut, ipNetIn)
 			}
 		})