The Rust port of `KNOWN_ROOM_VERSIONS` (#19589) made `__contains__`
strict about key types, raising `TypeError` when called with `None`
instead of returning `False` like a Python dict would.
This broke `/sync` for rooms with a NULL `room_version` in the database.
```
File "/home/synapse/src/synapse/handlers/sync.py", line 2628, in _get_room_changes_for_initial_sync
if event.room_version_id not in KNOWN_ROOM_VERSIONS:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: argument 'key': 'NoneType' object cannot be cast as 'str'
```
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