fix uploading when ['dirtoken'] is not set

apps/files/ajax/upload.php

@@ -8,7 +8,7 @@ OCP\JSON::setContentTypeHeader('text/plain');
 // If no token is sent along, rely on login only
 
 $l = OC_L10N::get('files');
-if (!$_POST['dirToken']) {
+if (empty($_POST['dirToken'])) {
 	// The standard case, files are uploaded through logged in users :)
 	OCP\JSON::checkLoggedIn();
 	$dir = isset($_POST['dir']) ? $_POST['dir'] : "";