Show 404 page when accessing empty share URL

Testplan:
- [ ] Without: Accessing `public.php?service=files&t=` throws an exception
- [ ] With: No exception thrown and 404 page displayed.

Fixes https://github.com/owncloud/core/issues/14231

apps/files_sharing/public.php

@@ -25,4 +25,10 @@ $urlGenerator = new \OC\URLGenerator(\OC::$server->getConfig());
 $token = isset($_GET['t']) ? $_GET['t'] : '';
 $route = isset($_GET['download']) ? 'files_sharing.sharecontroller.downloadShare' : 'files_sharing.sharecontroller.showShare';
 
-OC_Response::redirect($urlGenerator->linkToRoute($route, array('token' => $token)));
+if($token !== '') {
+	OC_Response::redirect($urlGenerator->linkToRoute($route, array('token' => $token)));
+} else {
+	header('HTTP/1.0 404 Not Found');
+	$tmpl = new OCP\Template('', '404', 'guest');
+	print_unescaped($tmpl->fetchPage());
+}