Fix error when `posix_getpwuid` returns false

The posix_getpwuid function can return false if it fails (see [docs](https://www.php.net/manual/en/function.posix-getpwuid.php#:~:text=The%20function%20returns%20false%20on%20failure.)).

src/Core/TempFolder.php

@@ -39,7 +39,7 @@ class TempFolder
         $user = '';
         if (function_exists('posix_getpwuid') && function_exists('posix_getuid')) {
             $userDetails = posix_getpwuid(posix_getuid());
-            $user = $userDetails['name'];
+            $user = $userDetails['name'] ?? false;
         }
         if (!$user) {
             $user = Environment::getEnv('APACHE_RUN_USER');