fix auth for file handler

metadata/metadata.go

@@ -9,6 +9,9 @@ import (
 type mapMetadata map[string]any
 
 func NewMetadata(m map[string]any) metadata.Metadata {
+	if len(m) == 0 {
+		return nil
+	}
 	md := make(map[string]any)
 	for k, v := range m {
 		md[strings.ToLower(k)] = v