Fix the context complaint differently

Author: David Robertson

synapse/storage/databases/main/devices.py

@@ -1865,6 +1865,8 @@ async def add_device_list_outbound_pokes(
 
         Marks the associated row in `device_lists_changes_in_room` as handled.
         """
+        if context is None:
+            context = {}
 
         def add_device_list_outbound_pokes_txn(
             txn: LoggingTransaction, stream_ids: List[int]
@@ -1876,7 +1878,9 @@ def add_device_list_outbound_pokes_txn(
                     device_ids=[device_id],
                     hosts=hosts,
                     stream_ids=stream_ids,
-                    context=context,
+                    # mypy doesn't know that `context` is never None here, see
+                    # https://mypy.readthedocs.io/en/stable/common_issues.html#narrowing-and-inner-functions
+                    context=context,  # type: ignore[arg-type]
                 )
 
             self.db_pool.simple_update_txn(