room stays unread if no messages due to retention policy

This issue has been migrated from [#10787](https://github.com/matrix-org/synapse/issues/10787).

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Initial report here: https://github.com/vector-im/element-web/issues/14976

### Description

If older messages gets removed due to retention policies and no new messages after retention time, room becomes unread forever until new message is inserted.

Tested clients: Element (web, Android and iOS)

### Steps to reproduce

- make new room
- set retention policy to short time
- send message and wait until retention kicks in
- clear client's cache and reload
- room stays unread

### Version information

- **Homeserver**: kvlt.ee

- **Version**: `{"server_version":"1.41.1","python_version":"3.7.3"}`

- **Install method**: pip

- **Platform**: Debian Buster