Gate finding B08-2: `((room_data or {}).get("timeline", {}) or {}).get("events", ()) or ()`
guarded against shapes a /sync join entry never has. rooms_join comes from
`sync_data.get("rooms", {}).get("join", {})` (same unguarded style a few lines
up) and mautrix's own handle_sync reads the identical payload as
`room_data.get("timeline", {}).get("events", [])`, so any malformed entry would
already fail there. Match that form; keep the per-event isinstance guard.
07030cc2d3