Stop trying to fetch events with event_id=None. (#5753)

`None` is not a valid event id, so queuing up a database fetch for it seems
like a silly thing to do.

I considered making `get_event` return `None` if `event_id is None`, but then
its interaction with `allow_none` seemed uninituitive, and strong typing ftw.

changelog.d/5753.misc

@@ -0,0 +1 @@
+Stop trying to fetch events with event_id=None.

synapse/handlers/message.py

@@ -378,7 +378,11 @@ class EventCreationHandler(object):
             # tolerate them in event_auth.check().
             prev_state_ids = yield context.get_prev_state_ids(self.store)
             prev_event_id = prev_state_ids.get((EventTypes.Member, event.sender))
-            prev_event = yield self.store.get_event(prev_event_id, allow_none=True)
+            prev_event = (
+                yield self.store.get_event(prev_event_id, allow_none=True)
+                if prev_event_id
+                else None
+            )
             if not prev_event or prev_event.membership != Membership.JOIN:
                 logger.warning(
                     (
@@ -521,6 +525,8 @@ class EventCreationHandler(object):
         """
         prev_state_ids = yield context.get_prev_state_ids(self.store)
         prev_event_id = prev_state_ids.get((event.type, event.state_key))
+        if not prev_event_id:
+            return
         prev_event = yield self.store.get_event(prev_event_id, allow_none=True)
         if not prev_event:
             return

synapse/storage/events_worker.py

@@ -139,8 +139,11 @@ class EventsWorkerStore(SQLBaseStore):
                 If there is a mismatch, behave as per allow_none.
 
         Returns:
-            Deferred : A FrozenEvent.
+            Deferred[EventBase|None]
         """
+        if not isinstance(event_id, str):
+            raise TypeError("Invalid event event_id %r" % (event_id,))
+
         events = yield self.get_events_as_list(
             [event_id],
             check_redacted=check_redacted,

synapse/storage/stats.py

@@ -211,16 +211,18 @@ class StatsStore(StateDeltasStore):
             avatar_id = current_state_ids.get((EventTypes.RoomAvatar, ""))
             canonical_alias_id = current_state_ids.get((EventTypes.CanonicalAlias, ""))
 
+            event_ids = [
+                join_rules_id,
+                history_visibility_id,
+                encryption_id,
+                name_id,
+                topic_id,
+                avatar_id,
+                canonical_alias_id,
+            ]
+
             state_events = yield self.get_events(
-                [
+                [ev for ev in event_ids if ev is not None]
-                    join_rules_id,
-                    history_visibility_id,
-                    encryption_id,
-                    name_id,
-                    topic_id,
-                    avatar_id,
-                    canonical_alias_id,
-                ]
             )
 
             def _get_or_none(event_id, arg):