Unable to create custom subscriptions
### Summary

Unable to create custom subscriptions using `makeExtendSchemaPlugin`. The websocket connection closes with the following error when I try to run the subscription query:
```
Connection Closed: 4500 Subscription field must return Async Iterable. Received: undefined.
```

Live queries work without issue when enabled. Enabling / disabling live queries and simple subscriptions doesn't seem to have any effect.

### Steps to reproduce

1. Enable `subscriptions: true`
2. Add `@graphile/pg-pubsub` to plugin hook
3. Add a custom subscription (in my case I've tried completely custom and using the example from https://www.graphile.org/postgraphile/subscriptions/)

### Expected results

The subscription should connect without issue.

### Actual results

The websocket is closed and the above error is thrown, then GraphiQL

### Additional context

- Node: 14-alpine
- Postgres: 12
- postgraphile: 4.12.4
- @graphile/pg-pubsub: 4.12.1
- @graphile/subscriptions-lds: 4.12.1
- @graphile-contrib/pg-simplify-inflector: 6.1.0
- postgraphile-plugin-connection-filter: 2.2.2
- postgraphile-plugin-nested-mutations: 1.1.0
- postgraphile-plugin-upload-field: 1.0.0-alpha.10
