fix: avoid NPE when previewing images with redis cache

CacheServiceRedisImpl#getImgCache(String) passed the key straight to Redisson, which throws NullPointerException (map key can't be null) for a null key. Every single-image preview hits this path because compressFileKey is only set for images inside an archive, so with cache.type=redis the preview endpoint always returned HTTP 500. Return an empty list for empty keys, matching CacheServiceJDKImpl.

Fixes #793

server/src/main/java/cn/keking/service/cache/impl/CacheServiceRedisImpl.java

@@ -1,12 +1,14 @@
 package cn.keking.service.cache.impl;
 
 import cn.keking.service.cache.CacheService;
+import org.apache.commons.lang3.StringUtils;
 import org.redisson.api.RBlockingQueue;
 import org.redisson.api.RMapCache;
 import org.redisson.api.RedissonClient;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnExpression;
 import org.springframework.stereotype.Service;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
@@ -68,6 +70,9 @@ public class CacheServiceRedisImpl implements CacheService {
 
     @Override
     public List<String> getImgCache(String key) {
+        if (StringUtils.isEmpty(key)) {
+            return new ArrayList<>();
+        }
         RMapCache<String, List<String>> convertedList = redissonClient.getMapCache(FILE_PREVIEW_IMGS_KEY);
         return convertedList.get(key);
     }