Keep a selection of only __typename as a struct, not an empty enum

[antonshevchenko]

A union member selected with only __typename generates an enum with no variants. Nothing can deserialize into it.

mutation Example {
  doThing {
    __typename
    ... on DoThingSuccess { __typename }
  }
}

DoThingSuccess carries no fields, so __typename is the only thing there is to select. The generated payload type comes out as pub enum ExampleDoThingOnDoThingSuccess {}, and any response naming that member fails to decode.

The cause is in ExpandedSelection::render, where a selection with no fields is turned into the enum of its variants. A selection of only __typename has no fields and no variants, so it becomes an empty enum. Requiring at least one variant leaves it as a struct, which deserializes from any object.