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
