driver/swift: treat empty object list as a PathNotFoundError

Swift returns an empty object list when trying to read a non-existing object path, treat it as a
PathNotFoundError when trying to list a non existing virtual directory.

Signed-off-by: David li <wenquan.li@hpe.com>

registry/storage/driver/swift/swift.go

@@ -589,7 +589,7 @@ func (d *driver) List(ctx context.Context, path string) ([]string, error) {
 		files = append(files, strings.TrimPrefix(strings.TrimSuffix(obj.Name, "/"), d.swiftPath("/")))
 	}
 
-	if err == swift.ContainerNotFound {
+	if err == swift.ContainerNotFound || (len(objects) == 0 && path != "/") {
 		return files, storagedriver.PathNotFoundError{Path: path}
 	}
 	return files, err