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