fix: set body to null if given empty buffer

[tompuric]

Raised for: https://github.com/lifeomic/axios-fetch/issues/127

Here's a PR that verifies the body is null. If the result data is an empty buffer, pass null as body instead. Ideally I would like to verify the node16-18 behaviour but the tests here hard-code the fetch and Response implementation to utilise node-fetch which behaves differently to node18s native fetch that utilises undici.