Zoekt shouldn't attempt indexing empty repositories

https://new-sentry.gitlab.net/organizations/gitlab/issues/119436/?project=3&query=is%3Aunresolved+Zoekt&referrer=issue-stream&statsPeriod=14d

RuntimeError Sidekiq/Zoekt::IndexerWorker
command [zoekt-git-index -index /data/index /data/repos/17971433.git] failed: exit status 1

We're seeing such failures when Zoekt::IndexerWorker attemps to index an empty repository. An example: https://gitlab.com/gitlab-org/monitor/sandbox/splattael-k8s-1-16