fix: stop GetIP erroring when the IP flag has a nil default

ip.go

@@ -32,6 +32,9 @@ func (i *ipValue) Type() string {
 }
 
 func ipConv(sval string) (interface{}, error) {
+	if sval == "" || sval == "<nil>" {
+		return net.IP(nil), nil
+	}
 	ip := net.ParseIP(sval)
 	if ip != nil {
 		return ip, nil

ip_nil_default_test.go

@@ -0,0 +1,35 @@
+package pflag
+
+import (
+	"net"
+	"testing"
+)
+
+func TestGetIPNilDefault(t *testing.T) {
+	t.Run("IP", func(t *testing.T) {
+		f := NewFlagSet("test", ContinueOnError)
+		f.IP("ip", nil, "IP address")
+
+		ip, err := f.GetIP("ip")
+		if err != nil {
+			t.Fatalf("GetIP returned error: %v", err)
+		}
+		if ip != nil {
+			t.Fatalf("expected nil IP, got %v", ip)
+		}
+	})
+
+	t.Run("IPVar", func(t *testing.T) {
+		f := NewFlagSet("test", ContinueOnError)
+		var ip net.IP
+		f.IPVar(&ip, "ip", nil, "IP address")
+
+		got, err := f.GetIP("ip")
+		if err != nil {
+			t.Fatalf("GetIP returned error: %v", err)
+		}
+		if got != nil {
+			t.Fatalf("expected nil IP, got %v", got)
+		}
+	})
+}