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