Fix an issue with rendering PHID lists containing `null` in Maniphest

Summary: See IRC. Someone got a `null` in CCPHIDs somehow. Moving to subscriptions should prevent this, but paper over it for now.

Test Plan: Will have @dctrwatson check.

Reviewers: btrahan, dctrwatson

Reviewed By: btrahan

CC: dctrwatson, aran

Differential Revision: https://secure.phabricator.com/D7330