R1:6d5f265a5795

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.