Federation - Public room: Join-over-federation failed when all the local members left it

This issue has been migrated from [#9555](https://github.com/matrix-org/synapse/issues/9555).

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### Description

A public room is not joinable anymore over the federation when it is left by all the local members.
Existing members can still send messages.
The visibility of the room is the rooms directory is lost too (but this point is a consequence, I think)

### Steps to reproduce

- Consider 2 users userA and userB hosted on a homeserver H1
- Consider 2 users userC and userD hosted on another homeserver H2
- userA creates a room public (join_rule: public, visibility: public)
- userB and userC join the room
- they send some messages
- userA and userB leave the room
- userC is alone, he shares the room link to userD
- userD failed to join the room from the link: errcode: "M_NOT_FOUND" error: "Not an active room on this server NOK
- the room is not visible anymore in the rooms directory NOK?
- userC shares the link to userA
- userA joins the room, then userD succeeds to join the room by using the link
- the room is again visible in the rooms directory