avoid empty extended attribute value

weed/server/filer_server_handlers_write_autochunk.go

@@ -218,7 +218,7 @@ func (fs *FilerServer) saveMetaData(ctx context.Context, r *http.Request, fileNa
 	entry.Extended = SaveAmzMetaData(r, entry.Extended, false)
 
 	for k, v := range r.Header {
-		if len(v) > 0 && (strings.HasPrefix(k, needle.PairNamePrefix) || k == "Cache-Control" || k == "Expires" || k == "Content-Disposition") {
+		if len(v) > 0 && len(v[0]) > 0 && (strings.HasPrefix(k, needle.PairNamePrefix) || k == "Cache-Control" || k == "Expires" || k == "Content-Disposition") {
 			entry.Extended[k] = []byte(v[0])
 		}
 	}