Don't return error when repos have no chart

Fix helm#632
I build a chartmuseum service which has no charts, then add the
chartmuseum repo into kubeapps. But I get a 'error' pod, that is
unfriendly. I think it is better to add log for it.

cmd/chart-repo/utils.go

@@ -99,7 +99,8 @@ func syncRepo(dbSession datastore.Session, repoName, repoURL string, authorizati
 
 	charts := chartsFromIndex(index, r)
 	if len(charts) == 0 {
-		return errors.New("no charts in repository index")
+		log.Warnf("The repository %s has no charts.", repoName)
+		return nil
 	}
 	err = importCharts(dbSession, charts)
 	if err != nil {