fix(repository): 修复人脸缓存获取逻辑

- 添加空值检查避免解析空字符串
- 提取缓存键避免重复格式化
- 优化缓存命中时的对象转换逻辑

src/main/java/com/ycwl/basic/repository/FaceRepository.java

@@ -30,7 +30,11 @@ public class FaceRepository {
 
     public FaceEntity getFace(Long id) {
         if (redisTemplate.hasKey(String.format(FACE_CACHE_KEY, id))) {
-            return JacksonUtil.parseObject(redisTemplate.opsForValue().get(String.format(FACE_CACHE_KEY, id)), FaceEntity.class);
+            String json = redisTemplate.opsForValue().get(String.format(FACE_CACHE_KEY, id));
+            if (json == null) {
+                return null;
+            }
+            return JacksonUtil.parseObject(json, FaceEntity.class);
         }
         FaceEntity face = faceMapper.get(id);
         if (face != null) {