show notification when renaming a file to ''

apps/files/js/filelist.js

@@ -151,6 +151,10 @@ var FileList={
 			var newname=input.val();
 			if (Files.containsInvalidCharacters(newname)) {
 				return false;
+			} else if (newname.length == 0) {
+				$('#notification').text(t('files', "Name cannot be empty."));
+				$('#notification').fadeIn();
+				return false;
 			}
 			if (newname != name) {
 				if (FileList.checkName(name, newname, false)) {