Avoid generating state groups for local out-of-band leaves (#12154)

If we locally generate a rejection for an invite received over federation, it
is stored as an outlier (because we probably don't have the state for the
room). However, currently we still generate a state group for it (even though
the state in that state group will be nonsense).

By setting the `outlier` param on `create_event`, we avoid the nonsensical
state.

changelog.d/12154.misc

@@ -0,0 +1 @@
+Avoid generating state groups for local out-of-band leaves.

synapse/handlers/room_member.py

@@ -1736,8 +1736,8 @@ async def _generate_local_out_of_band_leave(
             txn_id=txn_id,
             prev_event_ids=prev_event_ids,
             auth_event_ids=auth_event_ids,
+            outlier=True,
         )
-        event.internal_metadata.outlier = True
         event.internal_metadata.out_of_band_membership = True
 
         result_event = await self.event_creation_handler.handle_new_client_event(