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

dio/CHANGELOG.md

@@ -5,7 +5,10 @@ See the [Migration Guide][] for the complete breaking changes list.**
 
 ## Unreleased
 
-*None.*
+- Fix `FusedTransformer` (the default transformer) throwing a `FormatException`
+  on an empty response body when a custom `responseDecoder` is set. It now
+  returns an empty result, consistent with `SyncTransformer` and
+  `BackgroundTransformer`.
 
 ## 5.10.0
 

dio/lib/src/transformers/fused_transformer.dart

@@ -92,7 +92,9 @@ class FusedTransformer extends Transformer {
       decodedResponse = null;
     }
 
-    if (isJsonContent && decodedResponse != null) {
+    if (isJsonContent &&
+        decodedResponse != null &&
+        decodedResponse.isNotEmpty) {
       // slow path decoder, since there was a custom decoder specified
       return jsonDecode(decodedResponse);
     } else if (customResponseDecoder != null) {

dio/test/transformer_test.dart

@@ -384,6 +384,34 @@ void main() {
       expect(jsonResponse, null);
     });
 
+    test(
+      'empty JSON body with a custom responseDecoder does not throw',
+      () async {
+        // A custom decoder that returns the utf8-decoded body, which is an
+        // empty string for an empty response. Without guarding against an
+        // empty decoded string, the fused transformer would feed '' into
+        // jsonDecode and throw a FormatException, unlike SyncTransformer and
+        // BackgroundTransformer which both return an empty string here.
+        String decoder(List<int> bytes, RequestOptions o, ResponseBody rb) =>
+            utf8.decode(bytes, allowMalformed: true);
+        final transformer = FusedTransformer();
+        final response = await transformer.transformResponse(
+          RequestOptions(
+            responseType: ResponseType.json,
+            responseDecoder: decoder,
+          ),
+          ResponseBody.fromBytes(
+            [],
+            200,
+            headers: {
+              Headers.contentTypeHeader: [Headers.jsonContentType],
+            },
+          ),
+        );
+        expect(response, '');
+      },
+    );
+
     test('transform the request using urlencode', () async {
       final transformer = FusedTransformer();