bail out early in on_new_receipts if no pushers (#4706)

changelog.d/4706.misc

@@ -0,0 +1 @@
+Avoid some redundant work when processing read receipts

synapse/push/pusherpool.py

@@ -140,6 +140,10 @@ class PusherPool:
 
     @defer.inlineCallbacks
     def on_new_notifications(self, min_stream_id, max_stream_id):
+        if not self.pushers:
+            # nothing to do here.
+            return
+
         try:
             users_affected = yield self.store.get_push_action_users_in_range(
                 min_stream_id, max_stream_id
@@ -155,6 +159,10 @@ class PusherPool:
 
     @defer.inlineCallbacks
     def on_new_receipts(self, min_stream_id, max_stream_id, affected_room_ids):
+        if not self.pushers:
+            # nothing to do here.
+            return
+
         try:
             # Need to subtract 1 from the minimum because the lower bound here
             # is not inclusive