The example for ActionCable subscription may contain a bug

Describe the bug

I figured this one out after a long session of debugging: if you skip the initial response from the GraphQL subscription then the example GraphqlChannel from the docs will fail to register the subscription and throw an error about missing field.

Versions

graphql version: 2.4.8
graphql-ruby-client version: 1.14.5

Steps to reproduce

Implement ActionCable integration exactly as in the documentation, then use useSubscribe from basic Apollo stack.

I can provide full repro if needed.

Expected behavior

No errors from Apollo library, subscription registers correctly

Actual behavior

Missing field 'subscriptionName' while writing result {}

The subscription fails to be attached properly.

Additional context

This error happens because the example implementation of GraphqlChannel by default converts data: nil into data: {} (due to .to_h) and then always calls transmit, pushing empty data in an unexpected format. This is in turned picked by ActionCableLink which checks if result.data exists but not if it's an empty object, passing it to Apollo, triggering the bug.

I am not sure if this is expected behaviour in any configuration (Relay?) so I did not send a pull request but the solution when using base Apollo is simply to:

transmit(payload) unless result['data'].empty?

This will avoid the initial, broken call while making all the subsequent subscription events work as expected.

Start with the ActionCable GraphqlChannel example in the linked GraphQL API documentation and inspect javascript_client/src/subscriptions/ActionCableLink.ts around line 49. Reproduce the Apollo useSubscribe flow described in the issue, then verify that the initial empty payload no longer causes the missing-field error while subsequent subscription events still register correctly.