R1:e7e585820b96

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…