Successful responses with null data are rejected as errors

[pimterry]

My API includes mutations that don't return any data. They're effectively requests for a low-priority async operation, and they return almost immediately with no information, by just returning null.

That's works fine and its perfectly valid GraphQL as far as I'm aware, but this library rejects these responses, due to

fetchql/src/index.js

Lines 192 to 196 in 07d08be

	// if all properties of data is 'null'
	const allDataKeyEmpty = Object.keys(data).every(key => !data[key]);
	if (allDataKeyEmpty) {
	return reject(errors);
	}

In that code, a successful response that has no errors but which returns a { myOperation: null } result will throw undefined.

I don't think this case should throw. Even if it does, it should probably throw something clearer than undefined.

[gucheen]

According to GraphQL's specification, in 7.1.1 https://spec.graphql.org/June2018/#sec-Data

if the operation was a mutation, this output will be an object of the schema’s mutation root type.

It's better to return the root type of mutation. I would like to respect the specification.