Prevent an error when the Drydock lease artifact does not exist

Summary: If a host artifact exists in a Harbormaster build, where the Drydock lease no longer exists, then an error will be raised because of the attempt to access an undefined index.  This changes the code to use `idx()` so that it correctly returns null instead.

Test Plan: Tested in production.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: joshuaspence, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13074

src/applications/harbormaster/storage/build/HarbormasterBuildArtifact.php

@@ -77,7 +77,7 @@ final class HarbormasterBuildArtifact extends HarbormasterDAO
           ->setViewer($viewer)
           ->withIDs(array($data['drydock-lease']))
           ->execute();
-        $lease = $leases[$data['drydock-lease']];
+        $lease = idx($leases, $data['drydock-lease']);
 
         return id(new PHUIObjectItemView())
           ->setObjectName(pht('Drydock Lease'))