"EmptyQueryTypeException: Invalid query object type" when only exposing a subscription

[itsandreramon]

Library Version
3.6.1

Describe the bug
Cannot create a Spring Boot server using only a single subscription. I need to provide also a query. Spring Boot crashes with the following exception:

com.expediagroup.graphql.exceptions.EmptyQueryTypeException: Invalid query object type - no valid queries are available

To Reproduce

@Component
class BookSubscription(
  private val bookService: BookService
) : Subscription {

  @GraphQLDescription("Returns a single book in a non-blocking way")
  fun findBookById(id: Int): Mono<Book> {
    return bookService.findBookById(id)
  }
}

Expected behavior
Should generate a schema that consists only of one subscription type like this:

schema {
  subscription: Subscription
}

[dariuszkuc]

Root query type is required as per GraphQL Spec.

See graphql/graphql-spec#631 for additional details.