Allow extending types that haven't been declared

[vektah]

This comes in handy for apollo federation, and generatlly when decomposing schema down into small files.

eg when defining user.schema you might have:

extend Query {
   user(id: ID!): User
}

type User {
   id: ID!
   name: String!
}

If your whole app is architected like that then you don't have any file that should own the root query/mutation/schema types, which would be empty anyway. Allowing them to be created on the first extension removes this little wrinkle.