Return null data when data is empty

[OlfaKaroui]

We encountered an issue where the returned response data is replaced by an empty object when it's null. This happens when response code is 200 and an error is returned. Some libraries don't accept this and according to the gql specs data should be either null or populated {}.

Example :
Backend returns this response to a query:

{
  "data": null,
  "errors": [
    {
      "extensions": {
        "category": "client"
      },
      "message": "error message",
    }
  ]
}

You can see that in this case, data is null. Gateway then replaces that null with {} and returns this response :

{
  "data": {},
  "errors": [
    {
      "extensions": {
        "category": "client"
      },
      "message": "error message",
    }
  ]
}

This pull request is meant to address that issue. Let me know what you think!

[AlecAivazis]

Thanks for submitting this and I appreciate the extra tests 😄

I'll kick off the tests now and merge it in once we get a 👍

[OlfaKaroui]

@AlecAivazis Thank you! Any updates about the status of the PR ? 😄

[AlecAivazis]

Thanks for the reminder!

edit: this is now published under v0.1.17