skip JWT if fileId is empty

related to https://github.com/chrislusf/seaweedfs/issues/1808

weed/server/master_server_handlers.go

@@ -136,6 +136,9 @@ func (ms *MasterServer) dirAssignHandler(w http.ResponseWriter, r *http.Request)
 }
 
 func (ms *MasterServer) maybeAddJwtAuthorization(w http.ResponseWriter, fileId string, isWrite bool) {
+	if fileId == "" {
+		return
+	}
 	var encodedJwt security.EncodedJwt
 	if isWrite {
 		encodedJwt = security.GenJwt(ms.guard.SigningKey, ms.guard.ExpiresAfterSec, fileId)