timeline: `EventTimelineItem::latest_edit_json` is always empty for events edited locally

[zecakeh]

How to reproduce

1. Send an message.
2. Edit that message.
3. Check EventTimelineItem::latest_edit_json().

Outcome

The latest_edit_json is always empty.

Expected

The latest_edit_json should be populated when we receive the remote echo.

This only occurs in edits sent locally. Editing the same message again from a different client populates latest_edit_json.