Only work out newly_joined_users for incremental sync

synapse/handlers/sync.py

@@ -699,8 +699,11 @@ class SyncHandler(object):
 
         # Now we want to get any newly joined users
         newly_joined_users = set()
+        if sync_result_builder.since_token:
             for joined_sync in sync_result_builder.joined:
-            it = itertools.chain(joined_sync.timeline.events, joined_sync.state.values())
+                it = itertools.chain(
+                    joined_sync.timeline.events, joined_sync.state.values()
+                )
                 for event in it:
                     if event.type == EventTypes.Member:
                         if event.membership == Membership.JOIN: