Expected Behavior when there are no possibleTypes

[MarkHerhold]

How is this library supposed to behave when there are no in possibleTypes, e.g. on an interface that extends nothing?

File detail.graphql

schema {
  query: Query
}

type Query {
  getDetail: Detail
}

interface Detail {
  name: String!
  value: Float
}

File output.d.ts

// lots of lines omitted...

export interface DetailTypeResolver<TParent = any> {
  (parent: TParent, context: any, info: GraphQLResolveInfo):  | Promise<>; // <--- notice empty return type and no Promise return type
}

Debugger after evaluating const introspectResult = await getIntrospectResult(); here shows no possibleTypes (empty array):

My question is what is the intended output or if this is a bug, what is the expected output?