Make more code conform to strict null checks (#10219

* Make more code conform to strict null checks

* Fix types

* Fix tests

* Fix remaining test assertions

* Iterate PR

test/components/views/settings/tabs/room/AdvancedRoomSettingsTab-test.tsx

@@ -47,7 +47,7 @@ describe("AdvancedRoomSettingsTab", () => {
         mocked(dis.dispatch).mockReset();
         mocked(room.findPredecessor).mockImplementation((msc3946: boolean) =>
             msc3946
-                ? { roomId: "old_room_id_via_predecessor", eventId: null }
+                ? { roomId: "old_room_id_via_predecessor" }
                 : { roomId: "old_room_id", eventId: "tombstone_event_id" },
         );
     });
@@ -136,7 +136,7 @@ describe("AdvancedRoomSettingsTab", () => {
             fireEvent.click(link);
             expect(dis.dispatch).toHaveBeenCalledWith({
                 action: Action.ViewRoom,
-                event_id: null,
+                event_id: undefined,
                 room_id: "old_room_id_via_predecessor",
                 metricsTrigger: "WebPredecessorSettings",
                 metricsViaKeyboard: false,