Fails to parse responses when `Content-Length` header is stripped by proxy

[brunolca]

openapi-fetch version

0.13.8

Description

When a response with an empty body passes through Cloudflare (or other proxies), the client attempts to parse the empty response as JSON and fails. This happens because the current logic incorrectly treats the absence of a Content-Length header as indicating content is present.

Reproduction

// Mock server that returns empty body without Content-Length header
import express from 'express';

const app = express();

app.post('/test', (req, res) => {
  res.removeHeader('Content-Length');
  res.status(200).end();
});

app.listen(3000);

Or simulate with fetch mock:

global.fetch = async () => new Response(null, {
  status: 200,
  headers: {} // No Content-Length header
});

// Now use openapi-fetch - it will fail trying to parse empty body as JSON

Expected result

The client should gracefully handle empty responses regardless of whether the Content-Length header is present or absent. When a response has no body, response.data should be undefined or an appropriate empty value without attempting to parse it as JSON.

Extra

• I’m willing to open a PR (see CONTRIBUTING.md)