Summary:
Depends on D19095. Ref T6203. Ref T13077. This column is nullable in an inconsistent way. Make it non-nullable.
Also clean up one more content query on the history view.
Test Plan: Ran migration, then created and edited documents without providing a descriptino or hitting `NULL` exceptions.
Maniphest Tasks: T13077, T6203
Differential Revision: https://secure.phabricator.com/D19096
