refactor: 可読性のため一部でArray.prototype.atを使うように (#11274)

* refactor: `Array.prototype.at`を使うように

* fixup! refactor: `Array.prototype.at`を使うように

packages/backend/src/queue/processors/ExportNotesProcessorService.ts

@@ -90,7 +90,7 @@ export class ExportNotesProcessorService {
 					break;
 				}
 
-				cursor = notes[notes.length - 1].id;
+				cursor = notes.at(-1)?.id ?? null;
 
 				for (const note of notes) {
 					let poll: Poll | undefined;