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