Make 'event.redacts' never raise. (#6771)

There are quite a few places that we assume that a redaction event has a
corresponding `redacts` key, which is not always the case. So lets
cheekily make it so that event.redacts just returns None instead.

tests/storage/test_redaction.py

@@ -398,3 +398,38 @@ class RedactionTestCase(unittest.HomeserverTestCase):
         self.get_success(
             self.store.get_event(first_redact_event.event_id, allow_none=True)
         )
+
+    def test_store_redacted_redaction(self):
+        """Tests that we can store a redacted redaction.
+        """
+
+        self.get_success(
+            self.inject_room_member(self.room1, self.u_alice, Membership.JOIN)
+        )
+
+        builder = self.event_builder_factory.for_room_version(
+            RoomVersions.V1,
+            {
+                "type": EventTypes.Redaction,
+                "sender": self.u_alice.to_string(),
+                "room_id": self.room1.to_string(),
+                "content": {"reason": "foo"},
+            },
+        )
+
+        redaction_event, context = self.get_success(
+            self.event_creation_handler.create_new_client_event(builder)
+        )
+
+        self.get_success(
+            self.storage.persistence.persist_event(redaction_event, context)
+        )
+
+        # Now lets jump to the future where we have censored the redaction event
+        # in the DB.
+        self.reactor.advance(60 * 60 * 24 * 31)
+
+        # We just want to check that fetching the event doesn't raise an exception.
+        self.get_success(
+            self.store.get_event(redaction_event.event_id, allow_none=True)
+        )