R1:835cd7f2706f

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.