ClientWriter does not handle optional interfaces properly of the root query.

I fixed the processing of the root query in the scope of the PR. If the output type of a field is an interface, the ClientWriter will generate two methods: def {x}(...) and def {x}Option(...).

Schema 1 (without root schema)

type Queries {
  node(id: ID!): Node
}
interface Node {
  id: ID!
}
type NodeA implements Node {
  id: ID!
  a: String
}
type NodeB implements Node {
  id: ID!
  b: Int
}

ClientWriter generates two methods: node(id: ID)(...) and nodeOption(id: ID)(...).

Schema 2 (with top-level schema definition)

+ schema {
+  query: Queries
+}
type Queries {
  node(id: ID!): Node
}
interface Node {
  id: ID!
}
type NodeA implements Node {
  id: ID!
  a: String
}
type NodeB implements Node {
  id: ID!
  b: Int
}

ClientWriter generates only one method: node(id: ID)(...).