perf(SysUserServiceImpl): 优化缓存策略解决空值问题

- 为 selectNickNameById 方法添加缓存条件判断，避免对空值进行缓存
- 设置缓存条件为 #userId != null，防止无效参数触发缓存操作
- 配置 unless 规则为 #result == null，确保空结果不被缓存
- 减少不必要的缓存存储空间占用
- 提升缓存命中率和系统响应效率
- 降低数据库查询压力

ruoyi-system/src/main/java/com/ruoyi/system/service/impl/SysUserServiceImpl.java

@@ -518,7 +518,7 @@ public class SysUserServiceImpl implements ISysUserService, UserService {
         return ObjectUtil.isNull(sysUser) ? null : sysUser.getUserName();
     }
 
-    @Cacheable(cacheNames = CacheNames.SYS_NICK_NAME, key = "#userId")
+    @Cacheable(cacheNames = CacheNames.SYS_NICK_NAME, key = "#userId" ,condition = "#userId != null", unless = "#result == null")
     @Override
     public String selectNickNameById(Long userId) {
         SysUser sysUser = baseMapper.selectOne(new LambdaQueryWrapper<SysUser>()