Remove "files" arg from download URL in public link page

When no files were specified for download, it means folder download.
In this case, no need to pass an empty "files" argument.

apps/files_sharing/js/public.js

@@ -158,9 +158,11 @@ OCA.Sharing.PublicApp = {
 					filename = JSON.stringify(filename);
 				}
 				var params = {
-					path: path,
-					files: filename
+					path: path
 				};
+				if (filename) {
+					params.files = filename;
+				}
 				return OC.generateUrl('/s/' + token + '/download') + '?' + OC.buildQueryString(params);
 			};