fix: avoid NPE when previewing images with redis cache

[baiyu-jujuc]

Problem

With cache.type=redis, previewing a single image always returns HTTP 500. PDF / Office / archive previews are unaffected.

java.lang.NullPointerException: map key can't be null
    at org.redisson.RedissonMap.checkKey(RedissonMap.java:448)
    at cn.keking.service.cache.impl.CacheServiceRedisImpl.getImgCache(CacheServiceRedisImpl.java:72)
    at cn.keking.service.FileHandlerService.getImgCache(FileHandlerService.java:119)
    at cn.keking.service.impl.PictureFilePreviewImpl.filePreviewHandle(PictureFilePreviewImpl.java:34)

Root cause

PictureFilePreviewImpl#filePreviewHandle passes fileAttribute.getCompressFileKey() into getImgCache(String). kkCompressfileKey is only present when previewing an image inside an archive, so for a plain image the key is null.

CacheServiceRedisImpl#getImgCache(String) forwards that key straight to Redisson, which rejects a null key with NullPointerException. CacheServiceJDKImpl#getImgCache(String) already guards against an empty key, so only the redis implementation is affected.
Fix

Return an empty list for empty keys, aligning the redis implementation with the JDK one (5 lines).

Verification

• Added CacheServiceRedisImplTests (Mockito, no real Redis required):
  • null key → empty list, and verifyNoInteractions(redissonClient)
  • empty key → empty list
  • present key → still reads from Redis
• Verified the tests fail before the fix (Tests run: 3, Errors: 2) and pass after (Tests run: 3, Failures: 0).
• End-to-end: GET /onlinePreview?url=<base64 image url> with cache.type=redis returned 500 before, 200 after.

Fixes 793

[klboke]

Reviewed head c1b9daa and traced PictureFilePreviewImpl -> FileHandlerService -> CacheServiceRedisImpl. Returning an empty list for null/empty archive keys matches the existing JDK cache behavior and avoids calling Redisson for an ordinary image preview; a present key still reads the existing map.

Validation: all 3 CacheServiceRedisImplTests pass locally, including verifyNoInteractions for null/empty keys and a populated archive key. Linux, Windows, and macOS packaging plus PR E2E are all green on this head. No blocking findings.