Always return a deferred from `get_current_state_deltas`. (#7019)

This currently causes presence notify code to log exceptions when there
is no state changes to process. This doesn't actually cause any problems
as we'd simply do nothing anyway.

changelog.d/7019.misc

+Port `synapse.handlers.presence` to async/await.

synapse/storage/data_stores/main/state_deltas.py

@@ -15,6 +15,8 @@
 
 import logging
 
+from twisted.internet import defer
+
 from synapse.storage._base import SQLBaseStore
 
 logger = logging.getLogger(__name__)
@@ -56,7 +58,7 @@ class StateDeltasStore(SQLBaseStore):
             # if the CSDs haven't changed between prev_stream_id and now, we
             # know for certain that they haven't changed between prev_stream_id and
             # max_stream_id.
-            return max_stream_id, []
+            return defer.succeed((max_stream_id, []))
 
         def get_current_state_deltas_txn(txn):
             # First we calculate the max stream id that will give us less than