"Must provide Document" Error
Hi there,
today I was trying to implement a Avatar Upload system for user profiles.
So I came across apollo-upload server and client.
After implementing it like shown in the examples I'm getting this error and dont know how to solve it
`Error: Must provide document
    at invariant (/Users/tjarkkuehl/Desktop/semtexxx.de-website/server/node_modules/graphql/jsutils/invariant.js:18:11)
    at Object.validate (/Users/tjarkkuehl/Desktop/semtexxx.de-website/server/node_modules/graphql/validation/validate.js:58:34)
    at doRunQuery (/Users/tjarkkuehl/Desktop/semtexxx.de-website/server/node_modules/apollo-server-express/node_modules/apollo-server-core/src/runQuery.ts:143:30)
    at /Users/tjarkkuehl/Desktop/semtexxx.de-website/server/node_modules/apollo-server-express/node_modules/apollo-server-core/src/runQuery.ts:69:39
    at <anonymous>`

# Server Setup

## Index 
```

import { apolloUploadExpress } from 'apollo-upload-server'

app.use(
    '/graphql',
    bodyParser.json(),
    apolloUploadExpress({uploadDir: './user_upload/'})
)
```

## Schema

```
input Upload {
    name: String!
    type: String!
    size: Int!
    path: String!
}

type Mutation {
    uploadAvatar(file: Upload!) : Boolean!
}
```

## Resolver

```
uploadAvatar: (parent, args, context) => {
    console.log('Worked!')
    return true
}
```

# Client Setup

## Index

```
import { createUploadLink } from 'apollo-upload-client'

const link = createUploadLink({
    uri: `http://${config.graphqlAddress}:${config.graphqlPort}/graphql`
})

export const apolloClient = new ApolloClient({
    link: from([
        link,
        authMiddleware,
        authAfterware
    ]),
    cache: new InMemoryCache(),
    connectToDevTools: true
})
```

## Profile.vue

```
<input class="select_file_action"
                           type="file"
                           @change="tryChangeAvatar($event)">

this.$apollo.provider.defaultClient.mutate({
    mutation: gql`
    mutation ($file: Upload!) {
        uploadAvatar(file: $file)
    }`,
    variables: {
        file: target.files[0]
    }
})
```