Rewrite profile URL with username

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PHP

In this code with mod_rewrite I can visit the profile of any user by typing his username after domain like example.com/john this will take us to john's profile. This works well however if I type any name which is not a username for existing user in my database like example.com/notauser I still get redirected to a empty profile page, though it has no profile pic or other info but all other buttons and links like followers, friends etc are still available. In such a case I want to redirected to login page or echo message user does not exist, but can't figure out how to do that.

<?php
if(!empty($_GET['username'])) { 
$username = $_GET['username'];
}
else if(!empty($_SESSION['username'])) { 
$username = $_SESSION['username'];
}
else { 
header("Location: login.php"); 
die();
}
?>

.htaccess file

RewriteEngine On
RewriteRule ^([a-zA-Z0-9_-]+)$ profile.php?username=$1