Have questions about isset and empty?

I was looking at session issues recently and needed to determine whether the user was logged in. I found that the following two methods can successfully determine whether the user is logged in.
The code is as follows:

<code>第一种：
if(empty($_SESSION)){

    echo '您还未登录，请<a href="login.php">登录</a><p>';

}else{

    echo '欢迎'.$_SESSION['uname'].' 这里是主页 ';

    echo '<a href="quit.php">退出</a><p>';
}

第二种：
if(!isset($_SESSION['uname'])){

    echo '您还未登录，请<a href="login.php">登录</a><p>';

}else{

    echo '欢迎'.$_SESSION['uname'].' 这里是主页 ';

    echo '<a href="quit.php">退出</a><p>';
}

</code>

Question: In the second judgment statement, if you only write $_SESSION instead of $_SESSION['uname'], an error will be reported. If you use the first method, whether it is $_SESSION['uname in the brackets after empty '] or $_SESSION are no problem, what is the reason? Which of the two methods is better? Or is it safer? Thank you