Fix "arc linters" if no engine is built or configured

Summary: `$engine` will be undefined here. This was just copy/paste derp, combined with me accidentally having an `.arclint` file in a place I didn't expect to have one when I tried to test this.

Test Plan: Ran `arc linters` in a totally bare, non-.arclint-having working copy

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9255