Why from_id, to_id, and type_id do not have the same type and default values?

https://github.com/deepcase/deeplinks/blob/94d5926fdfaa0003c4a9c28980840d66512fdc4f/migrations/1616701513782-links.ts#L19

```
CREATE TABLE ${SCHEMA}."${TABLE_NAME}" (
id bigint PRIMARY KEY,
from_id bigint DEFAULT 0,
to_id bigint DEFAULT 0,
type_id bigint NOT NULL);
```

As far as I understand at the moment, `from_id` and `to_id` are nullable, but have 0 as a default value.
Type is not nullable, but it has no default value.