PostGraphileRelayPreset bug: mutation with "null" foreign key value does not work (returns null)
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### Summary

In case of `PostGraphileRelayPreset` mutation with "null" fk value does not work (returns null)

### Steps to reproduce

1. Clone this repo: https://github.com/TitanFighter/ouch-my-finger
2.  Create simple two tables with foreign key:

```
DROP TABLE IF EXISTS contacts;
DROP TABLE IF EXISTS customers;

CREATE TABLE customers(
    id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
    customer_name VARCHAR(255) NOT NULL
);

CREATE TABLE contacts(
    id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
    phone VARCHAR(15) NOT NULL,
    customer_id INT REFERENCES customers (id)
);

INSERT INTO customers (customer_name) VALUES ('Some Name');
INSERT INTO contacts (phone, customer_id) VALUES  ('+1111111', 1);
```

3. Create Postgre envs:
```
export DATABASE_URL=postgres:///my_db
export DATABASE_SCHEMAS=app_public
```

4. Run server: `node server-express.mjs`

5. Run mutation:
```
mutation MyMutation {
  updateContact(
    input: {
      id: "WyJDb250YWN0IiwxXQ=="
      contactPatch: { phone: "1", customerByCustomerId: null }
    }
  ) {
    contact {
      id
      phone
      customerByCustomerId {
        id
        customerName
      }
    }
  }
}
```


### Expected results

```
{
  "data": {
    "updateContact": {
      "contact": {
        "id": "WyJDb250YWN0IiwxXQ==",
        "phone": "1",
        "customerByCustomerId": null
      }
    }
  }
}
```

### Actual results

```
{
  "data": {
    "updateContact": null
  }
}
```

### Possible Solution

`package-lock.json` created on 13.12.2023 with `"graphile": "^5.0.0-beta.11"` fixes the problem.