Soften checks on a very old Maniphest transactionmigration

Summary:
Ref T9464. If an ancient transaction doesn't have array values for whatever reason, we fail here.

Instead, just recover as gracefully as we can. We may get the transaction "wrong" in some sense, but this only impacts what is rendered in the transaction log.

Test Plan: This is nearly a year old and there's no real way to test it.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9464

Differential Revision: https://secure.phabricator.com/D14149