How to correctly get the payload on notifications?
<!-- Before opening an issue: if you use `npm` try running `yarn` instead; if
      you run `yarn`, please try running `npx yarn-deduplicate && yarn`. -->

<!-- You can also chat with us on Discord: http://discord.gg/graphile -->

### Summary

This is my sample database:
```
CREATE TYPE BLOGPOST_ENUM AS ENUM ('Technical', 'Agile');

CREATE TABLE writer (
    id SERIAL PRIMARY KEY ,
    name TEXT NOT NULL ,
    surname TEXT NOT NULL ,
    nick TEXT
);

CREATE TABLE blog_post (
    id SERIAL PRIMARY KEY ,
    title TEXT NOT NULL ,
    content TEXT NOT NULL ,
    blogpost_type BLOGPOST_ENUM NOT NULL ,
    writer_id SERIAL REFERENCES writer(id)
);

CREATE OR REPLACE FUNCTION blogpost_added_subscription()
RETURNS trigger AS
$$
BEGIN
	PERFORM pg_notify(
		'postgraphile:blogpost_added',
		json_build_object(
			'__node__' , json_build_array(
        'blog_post',
        new.id
			)
		)::text
	);

	RETURN NEW;
END;
$$ LANGUAGE 'plpgsql';

CREATE OR REPLACE TRIGGER notifiy_after_blogpost_insert
  AFTER INSERT ON blog_post
  FOR EACH ROW
  EXECUTE FUNCTION blogpost_added_subscription()
;
```
When running this:
```
subscription MySubscription {
  listen(topic: "blogpost_added") {
    relatedNodeId
    relatedNode {
      nodeId
      ... on BlogPost {
        id
        nodeId
        title
        writerId
      }
    }
  }
}
```
I get:
```
{
  "data": {
    "listen": {
      "relatedNodeId": "WyJibG9nX3Bvc3QiLDNd",
      "relatedNode": null
    }
  }
}
```
I wish to get info about the recently new record inserted, I think I'm missing something on your [webpage](https://www.graphile.org/postgraphile/subscriptions/#topic-prefix). How to fix it?

### Additional context

<!-- Please provide additional context such as environment versions (including
      operating system, Node, PostgreSQL, Redis, etc. as relevant), software
      versions (from `yarn.lock`) or similar, plugins in use, database schema
      if relevant, bundler (if any). -->
