Changed checking for a closed task in arc tasks to look for the isClosed property instead of a non-empty status

Summary: I changed the way arc tasks was checking to see if a maniphest task was closed from looking for a non-empty status to looking for the isClosed property submitted in D8731.

Test Plan: Run arc tasks and check that the tasks show open/closed correctly.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8732