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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changelog.d/17727.bugfix

@@ -0,0 +1 @@
+Fix a bug in SSS which could prevent /sync from working for certain user accounts.

synapse/storage/databases/main/stream.py

@@ -1469,6 +1469,10 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         recheck_rooms: Set[str] = set()
         min_token = end_token.stream
         for room_id, stream in uncapped_results.items():
+            if stream is None:
+                # Despite the function not directly setting None, the cache can!
+                # See: https://github.com/element-hq/synapse/issues/17726
+                continue
             if stream <= min_token:
                 results[room_id] = stream
             else:
@@ -1495,7 +1499,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
     @cachedList(cached_method_name="_get_max_event_pos", list_name="room_ids")
     async def _bulk_get_max_event_pos(
         self, room_ids: StrCollection
-    ) -> Mapping[str, int]:
+    ) -> Mapping[str, Optional[int]]:
         """Fetch the max position of a persisted event in the room."""
 
         # We need to be careful not to return positions ahead of the current