Improve remarkup metadata robustness against debris in the database

Summary:
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