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
