### Description
We always build a subscription root, even when there was no possible fields. This breaks some third party clients, as the spec does not allow empty types in the schema. This PR fixes this by changing the `buildSubscriptionParser` helper to return a `Maybe` value, and harmonizes / cleans places where we build the subscription root.
https://github.com/hasura/graphql-engine-mono/pull/2357
GitOrigin-RevId: 1aeae25e321eee957e7645c436d17e69207309fd