Stop writing empty strings to the ownerPHID column

Summary: If we're unassigning an owner from a task it should set the column to `NULL` rather than an empty string. Fixes T3239

Test Plan: Assigned and Unassigned a task. Make sure the db is doing as excpected. Ran the patch, checked the db.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3239

Differential Revision: https://secure.phabricator.com/D6017