fix: Manually backfill config_id UUIDs again

The previous backfills handled NULL entries;
this handles empty strings.

The difficulty here was that since the empty string in an invalid UUID
(as opposed to None/NULL which is just "blank"), attempts to instantiate
models would fail, given the invalid data. This meant we couldn't use
the Django ORM to handle the migration entirely; we need to craft some
raw SQL to work-around these checks and limitations.