pg: Check revision.extra_headers is not null.

Fixes a regression in 038a219f, as it made the converter expect a list.

When adding this column, we made it default to null instead of defaulting to an empty array, so existing records were initialized will null.

This commit migrates these nulls to empty arrays, then adds a constraint to enforce it in the future.

---

Migrated from D3805 (view on Phabricator)