Correctly unmarshal null UDT responses

When a UDT column is null it's data will be empty, when this is the
case set the unmarshal target to its zero value if possible and
return.

marshal.go

@@ -1381,8 +1381,15 @@ func unmarshalUDT(info TypeInfo, data []byte, value interface{}) error {
 		}
 	}
 
-	udt := info.(UDTTypeInfo)
+	if len(data) == 0 {
+		if k.CanSet() {
+			k.Set(reflect.Zero(k.Type()))
+		}
+
+		return nil
+	}
 
+	udt := info.(UDTTypeInfo)
 	for _, e := range udt.Elements {
 		size := readInt(data[:4])
 		data = data[4:]

udt_test.go

@@ -187,3 +187,57 @@ func TestUDT_Proto2error(t *testing.T) {
 		t.Fatalf("expected to get %v got %v", ErrorUDTUnavailable, err)
 	}
 }
+
+func TestUDT_NullObject(t *testing.T) {
+	if *flagProto < protoVersion3 {
+		t.Skip("UDT are only available on protocol >= 3")
+	}
+
+	session := createSession(t)
+	defer session.Close()
+
+	err := createTable(session, `CREATE TYPE udt_null_type(
+		name text,
+		owner text);`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = createTable(session, `CREATE TABLE udt_null_table(
+		id uuid,
+		udt_col frozen<udt_null_type>,
+
+		primary key(id)
+	);`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	type col struct {
+		Name  string `cql:"name"`
+		Owner string `cql:"owner"`
+	}
+
+	id := TimeUUID()
+	err = session.Query("INSERT INTO udt_null_table(id) VALUES(?)", id).Exec()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	readCol := &col{
+		Name:  "temp",
+		Owner: "temp",
+	}
+
+	err = session.Query("SELECT udt_col FROM udt_null_table WHERE id = ?", id).Scan(readCol)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if readCol.Name != "" {
+		t.Errorf("expected empty string to be returned for null udt: got %q", readCol.Name)
+	}
+	if readCol.Owner != "" {
+		t.Errorf("expected empty string to be returned for null udt: got %q", readCol.Owner)
+	}
+}