added missing isEmpty ajax file

apps/files_trashbin/ajax/isEmpty.php

+<?php
+
+/*
+ * Check if trash bin is empty to re-enable the deleted files button if needed
+ */
+
+OCP\JSON::checkLoggedIn();
+OCP\JSON::callCheck();
+
+$trashStatus = OCA\Files_Trashbin\Trashbin::isEmpty(OCP\User::getUser());
+
+OCP\JSON::success(array("data" => array("isEmpty" => $trashStatus)));
+
+