Adjust logic for deciding if trashbin empty

Any error in opening the trashbin directory returns 'true' for isEmpty.
An error is prevented by checking if the directory exists before trying to
open it.

apps/files_trashbin/lib/trashbin.php

@@ -921,13 +921,11 @@ class Trashbin {
 	public static function isEmpty($user) {
 
 		$view = new \OC\Files\View('/' . $user . '/files_trashbin');
-		$dh = $view->opendir('/files');
-		if (!$dh) {
-			return false;
-		}
-		while ($file = readdir($dh)) {
-			if ($file !== '.' and $file !== '..') {
-				return false;
+		if ($view->is_dir('/files') && $dh = $view->opendir('/files')) {
+			while ($file = readdir($dh)) {
+				if ($file !== '.' and $file !== '..') {
+					return false;
+				}
 			}
 		}
 		return true;