BaseEventIndexManager: Fix some type annotations.

src/BaseEventIndexManager.js

@@ -159,8 +159,8 @@ export default class BaseEventIndexManager {
      */
     async addHistoricEvents(
         events: [HistoricEvent],
-        checkpoint: CrawlerCheckpoint | null = null,
-        oldCheckpoint: CrawlerCheckpoint | null = null,
+        checkpoint: CrawlerCheckpoint | null,
+        oldCheckpoint: CrawlerCheckpoint | null,
     ): Promise<bool> {
         throw new Error("Unimplemented");
     }