Remove an undefined warning when not yet logged in

DokuWiki would generate warnings at the top of every page with PHP 8.2 due to referencing a possibly undefined array entry. This expands the check for whether the items in the array exist or not.

auth.php

@@ -77,7 +77,7 @@ class auth_plugin_authuffd extends DokuWiki_Auth_Plugin
 		global $USERINFO, $INPUT, $conf;
 		if (!empty($_SESSION[DOKU_COOKIE]['auth']) && $_SESSION[DOKU_COOKIE]['auth']['time'] < time() - $conf['auth_security_timeout'])
 			$this->clearSession();
-		if (!empty($_SESSION[DOKU_COOKIE]['auth']))
+		if (!empty($_SESSION[DOKU_COOKIE]['auth']) && !empty($_SESSION[DOKU_COOKIE]['auth']['info']))
 		{
 			$USERINFO['name'] = $_SESSION[DOKU_COOKIE]['auth']['info']['name'];
 			$USERINFO['mail'] = $_SESSION[DOKU_COOKIE]['auth']['info']['mail'];