fix: getUserId()方法在用户未登录时返回null而不是0

- 修改SecurityContextHolder.getUserId()方法，未登录时返回null
- 避免未登录状态与用户ID为0的情况混淆
- UserConstants.isAdmin()已有null检查，兼容此修改

ruoyi-common/ruoyi-common-core/src/main/java/com/ruoyi/common/core/context/SecurityContextHolder.java

@@ -53,7 +53,8 @@ public class SecurityContextHolder
 
     public static Long getUserId()
     {
-        return Convert.toLong(get(SecurityConstants.DETAILS_USER_ID), 0L);
+        String value = get(SecurityConstants.DETAILS_USER_ID);
+        return StringUtils.isEmpty(value) ? null : Convert.toLong(value);
     }
 
     public static void setUserId(String account)