fix(consensus-db): tolerate a missing data table during migration

[Dusk1e]

migrate_certificates and migrate_decided_blocks open their table inside a read transaction:

let mut next_height = if let Some((min_height, _)) = self
    .db
    .begin_read()?
    .open_table(CERTIFICATES_TABLE)?
    .first()?

open_table on a read transaction returns TableDoesNotExist when the table isn't there. The other two table migrations, migrate_undecided_blocks and migrate_pending_parts, open theirs inside a write transaction, which creates it, so those already treat a missing table as an empty one. The first two abort the whole migration instead.

A datadir can reach the migration before the tables exist. Db::new migrates, and only afterwards does Store::open call create_tables. needs_migration records "file exists, no schema version" as v0 and asks for a migration, and redb::Database::create makes the file before that version is ever written — so a first start interrupted between the two leaves a database that every later start rejects with:

Table(TableDoesNotExist("certificates"))

db migrate fails the same way, --dry-run included, since preview_migrate runs the same functions.

The change treats a missing table as empty in both, which is what the other two already do. The added test fails on main with TableDoesNotExist("certificates") and passes with the change; the rest of the crate's suite (107 tests) still passes.