addFile method Null safety
Implementation
Future<void> addFile(File file, [String? filename]) async {
final fileStream = InputFileStream(file.path);
final f = ArchiveFile.stream(
filename ?? file.path, file.lengthSync(), fileStream);
f.lastModTime = file.lastModifiedSync().millisecondsSinceEpoch ~/ 1000;
f.mode = file.statSync().mode;
_encoder.add(f);
await fileStream.close();
}