Thread hidden when root message deleted and messages are collapsed

### Steps to reproduce

1. Send a number of messages
2. Start a thread from a message in the middle
3. Delete all events (for the thread only the root event, not inside the thread)

### Outcome

#### What did you expect?

Thread root not in collapsed messages.

#### What happened instead?

Thread root also gets collapsed.
This makes it hard to find the thread from the timeline.


### Operating system

Ubuntu 22.04.2 LTS

### Application version

Element Nightly version: 2023022301 Olm version: 3.2.12

### How did you install the app?

https://packages.element.io/debian/

### Homeserver

_No response_

### Will you send logs?

No

The issue names no source file or test. Start by reproducing the steps in Element Web with a thread rooted in the middle of a message sequence, then inspect the collapsed-message and thread-root handling; done means the deleted root is not collapsed while the thread remains discoverable in the timeline.

Written by the indexing model from the issue text.