Empty db.array will not error, but generate invalid SQL

For postgres, an empty table passed to `db.array` will generate syntactically invalid SQL:

(shortened)

```moon
model\create tags: DB.array data.topics or {}
```

yields

```SQL
INSERT INTO "projects" (<...>, "tags", <...>) VALUES (<...>, ], <...>) RETURNING "id"
```

This fails because it is rejected by PG.