rooms table has a bunch of nullable columns

See https://github.com/matrix-org/synapse/pull/16612#discussion_r1388011390, but `rooms` has the following nullable columns:

* `is_public`
* `creator`
* `room_version`
* `has_auth_chain_index`

The latter two were added to the table which is why they're nullable, but I suspect we can fill those in / add a constraint now?