Fix error when deleting an empty list of refs

Import of gitlab projects silently falls with an error ArgumentError: 3:DeleteRefs: empty ExceptWithPrefix and Refs in Sidekiq.

In Gitaly:

	if len(req.ExceptWithPrefix) == 0 && len(req.Refs) == 0 { // You can't delete all refs
		return fmt.Errorf("empty ExceptWithPrefix and Refs")
	}

Continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18590.

Closes #45743 (closed)