provide empty string as alternative for HTTP_ACCEPT_LANGUAGE, fixes #4209

Closes #4209

Merge request studip/studip!3038

public/install.php

@@ -43,7 +43,7 @@ if (!function_exists('_')) {
 } else {
     require_once 'lib/language.inc.php';
 
-    foreach (explode(',', $_SERVER['HTTP_ACCEPT_LANGUAGE']) as $lang) {
+    foreach (explode(',', $_SERVER['HTTP_ACCEPT_LANGUAGE'] ?? '') as $lang) {
         [$lang, ] = explode(';', $lang);
         $lang = substr($lang, 0, 2);