refactor: barcode code to non nullable

lib/src/types/barcode.dart

@@ -8,7 +8,7 @@ import 'barcode_format.dart';
 class Barcode {
   Barcode(this.code, this.format, this.rawBytes);
 
-  final String? code;
+  final String code;
   final BarcodeFormat format;
 
   /// Raw bytes are only supported by Android.