Fixes failing test in CI: `tests.handlers.test_federation.PartialJoinTestCase.test_failed_partial_join_is_clean`
```
2024-08-22 18:57:22-0500 [-] synapse.metrics.background_process_metrics - 253 - ERROR - sync_partial_state_room-0 - Background process 'sync_partial_state_room' threw an exception
Traceback (most recent call last):
File "synapse/synapse/metrics/background_process_metrics.py", line 251, in run
return await func(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "synapse/synapse/handlers/federation.py", line 1842, in _sync_partial_state_room_wrapper
await self._sync_partial_state_room(
File "synapse/synapse/handlers/federation.py", line 1933, in _sync_partial_state_room
await self.state_handler.update_current_state(room_id)
File "synapse/synapse/state/__init__.py", line 554, in update_current_state
await self._storage_controllers.persistence.update_current_state(room_id)
File "synapse/synapse/storage/controllers/persist_events.py", line 491, in update_current_state
await self._event_persist_queue.add_to_queue(
File "synapse/synapse/storage/controllers/persist_events.py", line 245, in add_to_queue
res = await make_deferred_yieldable(end_item.deferred.observe())
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "synapse/synapse/storage/controllers/persist_events.py", line 288, in handle_queue_loop
ret = await self._per_item_callback(room_id, item.task)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "synapse/synapse/storage/controllers/persist_events.py", line 370, in _process_event_persist_queue_task
await self._update_current_state(room_id, task)
File "synapse/synapse/storage/controllers/persist_events.py", line 507, in _update_current_state
await self.persist_events_store._calculate_sliding_sync_table_changes(
File "synapse/synapse/storage/databases/main/events.py", line 624, in _calculate_sliding_sync_table_changes
assert most_recent_event_pos_results, (
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: We should not be seeing `None` here because we are still in the room (!room:example.com) and it should at-least have a join membership event that's keeping us here.
```
