code generator generating strange typescript union types

**Intended outcome:**

Generating types from a query should not include an "empty" type and union it with the actual requested type.

**Actual outcome:**

`apollo client:codegen` is generating an empty typescript object type. It is then unioned with the requested type from the query.

generated code:

```typescript
export interface PopularTopicsClients_group_Topic {} // here is the strange "empty" type

export interface PopularTopicsClients_group_Group_featuredTopics_edges_node {
id: string;
}

export interface PopularTopicsClients_group_Group_featuredTopics_edges {
node: PopularTopicsClients_group_Group_featuredTopics_edges_node;
threadCount: number;
}

export interface PopularTopicsClients_group_Group_featuredTopics {
edges: (PopularTopicsClients_group_Group_featuredTopics_edges | null)[];
}

export interface PopularTopicsClients_group_Group {
featuredTopics: PopularTopicsClients_group_Group_featuredTopics;
}

// This unioned type is strange because `PopularTopicsClients_group_Topic` has no fields
// and is not being requested in the query
export type PopularTopicsClients_group = PopularTopicsClients_group_Topic | PopularTopicsClients_group_Group;

export interface PopularTopicsClients {
group: PopularTopicsClients_group | null;
}

export interface PopularTopicsClientsVariables {
first?: number | null;
groupId: string;
}
```
The strange type is `export type PopularTopicsClients_group = PopularTopicsClients_group_Topic | PopularTopicsClients_group_Group;`

`PopularTopicsClients_group_Topic` is a type with no fields and it's not being requested in the query (see the repro below).
This makes it challenging to use in code because `group` is either the fields ive asked for `PopularTopicsClients_group_Group` or some empty type. However, this does not match what the server will be returning, nor the structure of the query so it's quite confusing. So using it in code requires a guard:

```typescript
if (isEmptyType(group)) {
throw new Error('this is just so typescript is happy...');
}
// now group is definitely `PopularTopicsClients_group_Group` so we can access the fields on it
// do stuff with `group.featuredTopics`
```

**How to reproduce the issue:**

Use the following `schema.graphql`:

```graphql
interface Node {
id: ID!
}

type Group implements Node {
id: ID!

featuredTopics(
first: Int
): GroupTopicConnection!
}

type Topic implements Node {
id: ID!
}

type GroupTopicConnection {
edges:[GroupTopicEdge]!

pageInfo: PageInfo!
}

type GroupTopicEdge {
node: Topic!

cursor: String!

threadCount: Int!
}

type PageInfo {
hasNextPage: Boolean!

hasPreviousPage: Boolean!

startCursor: String

endCursor: String
}

type Query {

node(
id: ID!
): Node

}
```

With the following query:

```graphql

query PopularTopicsClients($first: Int = 5, $groupId: ID!) {
group: node(id: $groupId) {
... on Group {
featuredTopics(first: $first) {
edges {
node {
id
}
threadCount
}
}
}
}
}
```

Using the following command:

```bash
$ npx apollo client:codegen --includes=queryPopularTopics.graphql --target typescript --no-addTypename --localSchemaFile="schema.graphql" --outputFlat src/types --customScalarsPrefix GraphQl --globalTypesFile="src/types/globalTypes.ts"
```

**Versions**
Tried it on:

`apollo/2.30.2 linux-x64 node-v10.22.0`

Start with schema.graphql and queryPopularTopics.graphql, then run the documented apollo client:codegen command against them. Inspect the generated TypeScript for the empty PopularTopicsClients_group_Topic interface and union. Done means the generated group type contains only the requested Group shape, without the empty union member.

Written by the indexing model from the issue text.