Only fetch thread participation for events with threads. (#12228)

We fetch the thread summary in two phases:

1. The summary that is shared by all users (count of messages and latest event).
2. Whether the requesting user has participated in the thread.

There's no use in attempting step 2 for events which did not return a summary
from step 1.

changelog.d/12228.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in v1.53.0 where an unnecessary query could be performed when fetching bundled aggregations for threads.

synapse/storage/databases/main/relations.py

@@ -857,7 +857,9 @@ async def get_bundled_aggregations(
         summaries = await self._get_thread_summaries(events_by_id.keys())
         # Only fetch participated for a limited selection based on what had
         # summaries.
-        participated = await self._get_threads_participated(summaries.keys(), user_id)
+        participated = await self._get_threads_participated(
+            [event_id for event_id, summary in summaries.items() if summary], user_id
+        )
         for event_id, summary in summaries.items():
             if summary:
                 thread_count, latest_thread_event, edit = summary