generate_room_entry breaks due to missing create event

[clokep]

See https://sentry.matrix.org/sentry/synapse-matrixorg/issues/226318/

There's at least one room on matrix.org (!puCGMpKizEgwloleEe:matrix.org) where the current_state_events for the room is missing the m.room.create event. There is a create event ($155394871468891LqYwt:matrix.org), but it seems to not be in current_state_events (current_state_events consists solely of m.room.member events.)

What we think happened is:

1. Everyone on matrix.org left the room.
2. New members joined via it's possible to join an empty room over federation, resulting in a split-brain room #6958

So the room is now in a broken state, we can probably:

1. Make generate_room_entry more robust (by skipping the room?)
2. Purge the room
3. Try to fix the current_state_events for the room

Note that this code seems to affect the room list, as well as listing rooms in groups/communities.

[clokep]

@richvdh reports that there seems to be ~5000 rooms that might be affected by this.

[DMRobertson]

Sounds like a duplicate of #9330?

[clokep]

Both filed by me, that was silly...