Fix Maniphest fatal if attached tasks are not an array

Summary: Fixes T3678. I think some very old rows may have a junk value here. This will be obsoleted by ApplicationTransactions and other modernization, most likely, so just fix it locally.

Test Plan: looked at a task

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3678

Differential Revision: https://secure.phabricator.com/D6677