Make ResponseDecoder return nullable String type

[huhx]

This PR make the ResponseDecoder return nullable String

In options.dart:

typedef ResponseDecoder = String Function(
    List<int> responseBytes, RequestOptions options, ResponseBody responseBody);

The usage of this function: src/transformer.dart

String? responseBody;
if (options.responseDecoder != null) {
  responseBody = options.responseDecoder!(
    responseBytes,
    options,
    response..stream = Stream.empty(),
  );
} else {
  responseBody = utf8.decode(responseBytes, allowMalformed: true);
}

As we can see the responseBody is nullable, it means ResponseDecoder function return String? may be make sense.

[AlexV525]

LGTM. Merging to the fork.

[AlexV525]

Hi @huhx. We've made our hardfork repo public and published a new version of dio, named diox.
The new package contains the PR of the fix.
Please refer to https://pub.dev/packages/diox/versions/5.0.0-dev.1 to use the fork.
You can also see why we're working for a hardfork at cfug/diox#29 and #1607.

[huhx]

Hi @huhx. We've made our hardfork repo public and published a new version of dio, named diox. The new package contains the PR of the fix. Please refer to https://pub.dev/packages/diox/versions/5.0.0-dev.1 to use the fork. You can also see why we're working for a hardfork at cfug/diox#29 and #1607.

Great works, thank you.