fix type

packages/backend/src/misc/cache.ts

@@ -8,7 +8,7 @@ export class RedisKVCache<T> {
 	private memoryCache: MemoryKVCache<T>;
 	private fetcher: (key: string) => Promise<T>;
 	private toRedisConverter: (value: T) => string;
-	private fromRedisConverter: (value: string) => T;
+	private fromRedisConverter: (value: string) => T | undefined;
 
 	constructor(redisClient: RedisKVCache<T>['redisClient'], name: RedisKVCache<T>['name'], opts: {
 		lifetime: RedisKVCache<T>['lifetime'];
@@ -92,7 +92,7 @@ export class RedisSingleCache<T> {
 	private memoryCache: MemorySingleCache<T>;
 	private fetcher: () => Promise<T>;
 	private toRedisConverter: (value: T) => string;
-	private fromRedisConverter: (value: string) => T;
+	private fromRedisConverter: (value: string) => T | undefined;
 
 	constructor(redisClient: RedisSingleCache<T>['redisClient'], name: RedisSingleCache<T>['name'], opts: {
 		lifetime: RedisSingleCache<T>['lifetime'];