Fix partial-stated room re-syncing state but nothing has changed

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.
```

synapse/storage/databases/main/events.py

@@ -178,7 +178,7 @@ class SlidingSyncTableChanges:
     # correct latest value.
     #
     # This should be *some* value that points to a real event in the room if we are
-    # still joined to the room.
+    # still joined to the room and some state is changing (`to_insert` or `to_delete`).
     joined_room_best_effort_most_recent_stream_ordering: Optional[int]
     # Values to upsert into `sliding_sync_joined_rooms`
     joined_room_updates: SlidingSyncStateInsertValues
@@ -394,6 +394,18 @@ class PersistEventsStore:
         to_insert = delta_state.to_insert
         to_delete = delta_state.to_delete
 
+        # If no state is changing, we don't need to do anything. This can happen when a
+        # partial-stated room is re-syncing the current state.
+        if not to_insert and not to_delete:
+            return SlidingSyncTableChanges(
+                room_id=room_id,
+                joined_room_best_effort_most_recent_stream_ordering=None,
+                joined_room_updates={},
+                membership_snapshot_shared_insert_values={},
+                to_insert_membership_snapshots=[],
+                to_delete_membership_snapshots=[],
+            )
+
         event_map = {event.event_id: event for event, _ in events_and_contexts}
 
         # Handle gathering info for the `sliding_sync_membership_snapshots` table