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

aik099/arcanist,chundongwang/arcanist,codelabs-ch/arcanist,zhihu/arcanist,dguo-coursera/arcanist,calblueprint/arcanist,folsom-labs/arcanist,fivestars/arcanist,eSpark/arcanist,eSpark/arcanist,zhihu/arcanist,coursera/arcanist,Symplicity/arcanist,eSpark/arcanist,wharris/arcanist,wharris/arcanist,zhihu/arcanist,memsql/arcanist,wxstars/arcanist,dguo-coursera/arcanist,PageUpPeopleOrg/arcanist-patches,PageUpPeopleOrg/arcanist-patches,aik099/arcanist,coursera/arcanist,uber/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,fivestars/arcanist,wikimedia/phabricator-arcanist,haskell-infra/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,KieranHayes/arcanist,kevinseelbach/arcanist,KieranHayes/arcanist,uber/arcanist,hach-que/arcanist,folsom-labs/arcanist,phacility/arcanist,coursera/arcanist,PageUpPeopleOrg/arcanist-patches,phacility/arcanist,haskell-infra/arcanist,hach-que/arcanist,Khan/arcanist,Symplicity/arcanist,memsql/arcanist,Symplicity/arcanist,wikimedia/phabricator-arcanist,dguo-coursera/arcanist,coursera/arcanist,r4nt/arcanist,KieranHayes/arcanist,codelabs-ch/arcanist,hach-que/arcanist,calblueprint/arcanist,kevinseelbach/arcanist,alexmv/arcanist,phacility/arcanist,dguo-coursera/arcanist,sneakpeeq/arcanist,wikimedia/phabricator-arcanist,wxstars/arcanist,kevinseelbach/arcanist,kevinseelbach/arcanist,wharris/arcanist,Khan/arcanist,aik099/arcanist,haskell-infra/arcanist,jra3/arcanist,chundongwang/arcanist,fivestars/arcanist,phacility/arcanist,fivestars/arcanist,Khan/arcanist,wharris/arcanist,r4nt/arcanist,memsql/arcanist,alexmv/arcanist,Symplicity/arcanist,sneakpeeq/arcanist,aik099/arcanist,sneakpeeq/arcanist,zhihu/arcanist,alexmv/arcanist,alexmv/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,chundongwang/arcanist,haskell-infra/arcanist,jra3/arcanist,eSpark/arcanist,chundongwang/arcanist,calblueprint/arcanist,wxstars/arcanist,uber/arcanist,uber/arcanist,jra3/arcanist,phacility/arcanist,jra3/arcanist,codelabs-ch/arcanist,Khan/arcanist,memsql/arcanist,folsom-labs/arcanist,KieranHayes/arcanist,sneakpeeq/arcanist,hach-que/arcanist,wxstars/arcanist,calblueprint/arcanist,r4nt/arcanist