fetch: following a 307 for a PUT with empty body fails

[macno]

Bug Description

following an error with octokit, we discovered that a fetch with method PUT (but I guess also other methods too) and an empty body {body: ''} that receives a 307 from the server, fails with the code AssertionError [ERR_ASSERTION]: '' == true

this is the error we see with octokit

RequestError [HttpError]: fetch failed
    at /app/node_modules/@octokit/request/dist-node/index.js:110:11
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async Job.doExecute (/app/node_modules/bottleneck/light.js:405:18) {
  status: 500,
  request: {
    method: 'PUT',
    url: 'https://api.github.com/repos/[REDACTED]/pulls/2764/merge',
    headers: {
      accept: 'application/vnd.github.v3+json',
      'user-agent': 'octokit.js/2.0.7 octokit-core.js/4.0.5 Node.js/18.13.0 (linux; x64)',
      authorization: 'token [REDACTED]'
    },
    body: '',
    request: {
      hook: [Function: bound bound register],
      retryCount: 3,
      retries: 3,
      retryAfter: 16
    }
  }
}

Reproducible By

> const headers = {
..       accept: 'application/vnd.github.v3+json',
...       'user-agent': 'octokit.js/2.0.14 octokit-core.js/4.2.0 Node.js/18.15.0 (darwin; arm64)',
...       authorization: 'token [REDACTED]' };
> await globalThis.fetch('https://api.github.com/[redacted]/pulls/2772/merge', { method: 'PUT', headers, body: '' })
Uncaught TypeError: fetch failed
    at Object.fetch (node:internal/deps/undici/undici:14062:11)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async REPL15:1:33 {
  cause: AssertionError [ERR_ASSERTION]: '' == true
      at httpRedirectFetch (node:internal/deps/undici/undici:13682:9)
      at httpFetch (node:internal/deps/undici/undici:13638:28)
      at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
      at async schemeFetch (node:internal/deps/undici/undici:13546:18)
      at async node:internal/deps/undici/undici:13422:20
      at async mainFetch (node:internal/deps/undici/undici:13418:20) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: '',
    expected: true,
    operator: '=='
  }
}

Expected Behavior

that fetch follows the redirect without any error

Logs & Screenshots

Environment

we see the error with the following envs:

• alpine 3.17 node 18.13 (amd64)
• alpine 3.17 node 18.15 (amd64 / arm64)
• macos 13.2.1 node 18.15 (arm64)

Additional context

Issue seems to be here because assert('') throws an error
I will propose to drop the assertion since a check that verifies body.source is not null is already present here

Happy to provide a PR if needed