Handle an empty argument

This makes it slightly easier to adjust VSCode's launch.json to hop around debugging different days. Not much, but a little. And every little bit helps!

main.go

@@ -33,7 +33,7 @@ var dayMap = []day{
 
 func main() {
 	arg := strconv.Itoa(len(dayMap))
-	if len(os.Args) > 1 {
+	if len(os.Args) > 1 && len(os.Args[1]) > 0 {
 		arg = os.Args[1]
 	}
 	if strings.ToLower(arg) == "all" {