Don't return error when repos have no chart #632

When I add a empty repos into kubeapps, then I get a error pod.

logs:
# kubectl logs apprepo-sync-chartmuseum-q97wv-jjsgh -n kube-system
time="2019-05-27T07:52:31Z" level=fatal msg="Can't add chart repository to database: no charts in repository index"
# kubectl get pod -n kube-system | grep apprepo-sync-chartmuseum
apprepo-sync-chartmuseum-q97wv-jjsgh 0/1 Error 1 51s