Prevent buildable list in Harbormaster from breaking when container or buildables are missing

Summary: Ref T1049. I'm fair sure this is just a case of bad data in my local install, but we probably don't want the default page for Harbormaster to break when there's invalid / missing container or buildable handles on any of the builds.

Test Plan: Loaded the page, didn't get a crash due to null reference.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: demo, epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8608