Fix for: Invalid requests generated for queries without input variables

[davjo664]

When generating requests for queries without any variables the query becomes:

static Json request() {
    Json query = R"(
        query Orders(
        ) {
            orders {
                id
                orderNo
            }
        }
    )";
    Json variables;
    return {{"query", std::move(query)}, {"variables", std::move(variables)}};
}

This results in an error when sending the query to my GraphQL API because of the empty variables input query Orders( ) { .... } it should be query Orders { ... }.