Clean up parentDomain issues in PhameBlog

Summary: Ref T9360. These weren't getting set properly, also make them nullable since they're optional.

Test Plan: run upgrade, make a new blog with and without a parent domain. Edit a current blog.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9360

Differential Revision: https://secure.phabricator.com/D16242