Timeline should keep back paginating if there are no visible events

Timeline should keep back paginating if there are no visible events.

Currently, after one `/messages` request without any visible events (for example a `/messages` response completely filled with `m.room.redaction`), it just gets stuck in this "spinner" state with no actual requests going on in the background.

This is from a real-life scenario in the `#synapse-dev:matrix.org` room:

These kinds of bugs are exactly why I advocate for a "Load more" button at the top of the timeline: https://github.com/element-hq/element-web/issues/21240 - Regardless of whether we fix infinite scrolling, an unlimited number of other problems can pop up and people just need to get unstuck.

Other related issue https://github.com/matrix-org/matrix-rust-sdk/issues/5600 when I thought this was a `matrix-rust-sdk` bug but seems to be just an `aurora` bug.

### Dev notes

Relevant code: https://github.com/element-hq/aurora/blob/5627193f9e6e89991483f2369c13eb8c18ec3b02/src/TimelineStore.tsx#L168-L180