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