fix: skip meta-only SSE events with empty data to prevent JSONDecodeError

[tarminik]

Summary

Fixes #2722

Per the SSE spec (WHATWG HTML § 9.2), SSE streams may contain meta-only events — events with only retry, id, or event fields but no data. Example: retry: 3000\n\n.

The SSEDecoder correctly parses these and emits ServerSentEvent with data="". However, Stream.__stream__() and AsyncStream.__stream__() unconditionally call sse.json() (which does json.loads("")), causing JSONDecodeError.

This happens in practice when streaming through gateways/proxies that inject SSE retry directives.

Fix

Added if not sse.data: continue in both sync and async __stream__() loops, right after the [DONE] check. Meta-only SSE events (empty data) are control frames and carry no user-facing payload — they are now silently skipped.

Tests

Added 3 new test cases (each parametrized for sync/async, 6 tests total) that exercise __stream__() directly:

1. Meta-only retry event followed by valid JSON — verifies no JSONDecodeError
2. Event-only frame (no data) interleaved with valid data — verifies only JSON events are yielded
3. thread.* event without data — verifies the thread-event branch also skips empty data

All existing tests continue to pass.

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