Bug: Accept empty types in GraphQL schemas

[krisbitney]

We should consider allowing the usage of empty types to support polymorphism.

For example, let's say I want this:

type Action { }
type CreateAccount implements Action { }
type DeployContract implements Action { 
  code: Bytes! 
}

Right now I'm required to do something like this:

type Action { 
  _: String
}

type CreateAccount implements Action { 
  _: String 
}

type DeployContract implements Action { 
  code: Bytes! 
}

Also note the _ placeholders propagate to generated TypeScript types.

Here is an example exception thrown when attempting to use empty types:

$ npx w3 plugin codegen
✔ Manifest loaded from ./web3api.plugin.yaml
✖ Failed to generate types: Syntax Error: Expected Name, found "}".
GraphQLError [Object]: Syntax Error: Expected Name, found "}".
    at syntaxError (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/error/syntaxError.js:15:10)
    at Parser.expectToken (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:1413:40)
    at Parser.parseName (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:98:22)
    at Parser.parseFieldDefinition (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:879:21)
    at Parser.optionalMany (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:1503:28)
    at Parser.parseFieldsDefinition (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:868:17)
    at Parser.parseObjectTypeDefinition (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:812:23)
    at Parser.parseTypeSystemDefinition (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:708:23)
    at Parser.parseDefinition (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:150:23)
    at Parser.many (/Users/kris/WebstormProjects/monorepo/node_modules/graphql/language/parser.js:1523:26) {
  locations: [ { line: 122, column: 1 } ]
}

Interestingly, this is in the same schema and appears to build without any problems. I actually get the opposite behavior: when I try to add the _ field to the child type, I get an exception complaining about duplicate fields in the type.

type AccessKeyPermission {
  _: String
}

type FullAccessPermission implements AccessKeyPermission {}