fix(dio): avoid FormatException on empty JSON body with a custom responseDecoder

[tonytonycoder11]

Problem

FusedTransformer is the default Transformer. When a request sets a custom responseDecoder and the server returns an empty body with a JSON content-type (a normal 200/204/304), the transformer's slow path runs jsonDecode(''), throwing FormatException: Unexpected end of input.

SyncTransformer and BackgroundTransformer handle the identical input gracefully — they guard the empty string and return it (which dio normalizes to null for typed JSON responses). Only the default transformer crashes.

This is the residual half of #2279: PR #2285 fixed the empty-body → null behavior for the fast path (no custom decoder) but never guarded the custom-decoder slow path.

Fix

Add an isNotEmpty check to the slow-path guard so an empty decoded body falls through to the existing return decodedResponse; branch, matching SyncTransformer / BackgroundTransformer:

if (isJsonContent && decodedResponse != null && decodedResponse.isNotEmpty) {
  return jsonDecode(decodedResponse);
} else if (customResponseDecoder != null) {
  return decodedResponse;
}

Non-empty malformed bodies still throw, exactly as before and as the sibling transformers do (the fix is intentionally scoped to match their isNotEmpty-only guard).

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary) — n/a, no public API change
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

Additional context and info (if any)

Reproduction (no network needed), calling the default transformer directly:

String decoder(List<int> b, RequestOptions o, ResponseBody rb) =>
    utf8.decode(b, allowMalformed: true);

await FusedTransformer().transformResponse(
  RequestOptions(responseType: ResponseType.json, responseDecoder: decoder),
  ResponseBody.fromBytes([], 200, headers: {
    Headers.contentTypeHeader: [Headers.jsonContentType],
  }),
);
// before: throws FormatException: Unexpected end of input
// after:  returns '' (consistent with SyncTransformer / BackgroundTransformer)

The added regression test (dio/test/transformer_test.dart → "empty JSON body with a custom responseDecoder does not throw") fails without the fix and passes with it.

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
dio/lib/src/transformers/fused_transformer.dart	🟢 93.88%	🟢 94%	🟢 0.12%
Overall Coverage	🟢 85.55%	🟢 85.56%	🟢 0.01%

Minimum allowed coverage is 0%, this run produced 85.56%