Improve remarkup metadata robustness against debris in the database
Ref T13682. Versioned drafts may have missing or unexpected metadata:
- versioned drafts from an older version of Phabricator may be missing metadata;
- versioned drafts created by an older UI against a newer version of Phabricator may have null metadata.
Generally, make these workflows robust to metadata in unexpected formats, so database debris doesn't break the UI.
Test Plan: Simulated debris, interacted with UI.
Maniphest Tasks: T13682
Differential Revision: https://secure.phabricator.com/D21849