re #45743 fix gitaly_delete_refs

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

gitaly validation for delete_refs:

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