DispatchInterceptor with POST request doesn't work

Bug Description

When a POST request is made using a dispatch interceptor and undici.DecoratorHandler, an error is thrown if the request includes a body. However, if the request does not include a body, no error is thrown.

/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/index.js:109
      Error.captureStackTrace(err, this)
            ^
TypeError: fetch failed
    at fetch (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/index.js:109:13) {
  cause: TypeError: this.handler.onBodySent is not a function
      at undiciInterceptorTesto.onBodySent (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/handler/DecoratorHandler.js:33:25)
      at Request.onBodySent (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/core/request.js:205:24)
      at AsyncWriter.write (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/client.js:1714:13)
      at writeIterable (/Users/battal/Desktop/files/projects/win-bot/node_modules/undici/lib/client.js:1637:19)
}

Reproducible By

import * as undici from "undici";
import { fetch } from "undici";

const undiciInterceptorHeaders = (dispatch: any) => {
  class undiciInterceptorTesto extends undici.DecoratorHandler {
    onHeaders(statusCode: any, headers: any, resume: any) {
      // @ts-ignore
      return super.onHeaders(statusCode, headers, resume);
    }
  }
  return function InterceptedDispatch(opts: any, handler: any) {
    return dispatch(opts, new undiciInterceptorTesto(handler));
  };
};

let dispatcher = new undici.Agent({
  interceptors: {
    Client: [undiciInterceptorHeaders as unknown as undici.Dispatcher],
  },
});

fetch("https://postman-echo.com/post", {
  method: "POST",
  body: new URLSearchParams("amk=5&asdm=6"),
  // body: new URLSearchParams(""), // works fine
  dispatcher,
})
  .then((res) => res.json())
  .then((body) => {
    console.log(body);
  });

Expected Behavior

it should not throw error

Environment

Macos Sonoma Beta 4
Node v20.5.0

Start with lib/handler/DecoratorHandler.js and follow the onBodySent path into lib/core/request.js and lib/client.js. Run the TypeScript reproducer with a POST body and compare it with the empty-body case. Done means the dispatch interceptor handles the body-bearing POST without throwing the reported onBodySent error.