Accept null via conduit.edit to unassign a task

Summary:
See https://discourse.phabricator-community.org/t/maniphest-edit-to-unassign-owner-documentation-is-wrong/1053. This unusual field doesn't actually accept null, although the documentation says it does and that was the intent.

Accept null, and show phid|null in the docs.

Test Plan: Viewed docs, saw phid|null. Unassigned with null.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18976