Fix/sse empty data format exception

[vazarkevych]

fix: handle empty SSE data payload to prevent unhandled FormatException

Problem

When a GrowthBook SSE stream emits a heartbeat or keepalive event with an empty data field, the SDK crashed with an unhandled FormatException.

The root cause was a two-part issue:

1. sseModel.data ?? "" silently fell through to jsonDecode(""), which throws a FormatException on an empty string.
2. The SSE .listen() callback runs asynchronously, so the exception escaped the surrounding try/catch and became an unhandled error — crashing the listener silently in production.

Fix

In lib/src/Network/network.dart, the features event handler now guards against empty or null payloads before attempting JSON decoding:

final data = sseModel.data;
if (data == null || data.isEmpty) return;
Map<String, dynamic> jsonMap = jsonDecode(data);

Heartbeat/keepalive events are silently dropped. JSON parsing is only attempted when there is actual data.

Tests

Added test/common_test/sse_empty_data_test.dart covering:

• Empty data field (data: ) — no unhandled exception, onSuccess not called
• Missing data field (null) — no unhandled exception, onSuccess not called
• Valid JSON payload — onSuccess called with the parsed map

[madhuchavva]

Inline review comment on SSE malformed payload handling.

[madhuchavva]

This fixes the heartbeat/empty-data case, which is good. There is still one related robustness gap: non-empty malformed data can still throw inside this async .listen() callback. If the server sends truncated JSON, invalid JSON, or a payload shape that is not Map<String, dynamic>, jsonDecode(data) can escape the surrounding request try/catch and become an unhandled async error. Can we wrap decode/type validation locally and route failures through onError or intentionally ignore only known heartbeat/keepalive events?

[vazarkevych]

Fixed, wrapped jsonDecode and the type check in a local try/catch inside the .listen() callback — all parse failures now route through onError instead of escaping as unhandled async errors. Added tests for malformed JSON and non-object payloads.

[madhuchavva]

Requesting changes based on the inline review comments. The notes call out functional gaps or compatibility/documentation risks that should be addressed before merge.