fix(backend): deliverManyにcontentのnullチェックを追加

https://github.com/MisskeyIO/misskey/pull/99

packages/backend/src/core/QueueService.ts

@@ -97,6 +97,8 @@ export class QueueService {
 	 */
 	@bindThis
 	public async deliverMany(user: ThinUser, content: IActivity | null, inboxes: Map<string, boolean>) {
+		if (content == null) return null;
+
 		const opts = {
 			attempts: this.config.deliverJobMaxAttempts ?? 12,
 			backoff: {