Sliding Sync: bugfix: ensure we can sync with SSS even with missing rooms (#17727)

Fixes https://github.com/element-hq/element-x-ios/issues/3300

Some rooms are missing from `sliding_sync_joined_rooms`. When this
happens, the first call will succeed, but any subsequent calls for this
room ID will cause the cache to return `None` for the room ID, rather
than not having the key at all. This then causes the `<=` check to
throw.

Root cause: https://github.com/element-hq/synapse/issues/17726

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