Don't crash when a remote beforeRequest callback returns nothing

[philipp-winterle]

The problem

beforeRequest is documented and typed as returning nothing when a callback only
wants to inspect a request:

beforeRequest?: (req: CompletedRequest) => MaybePromise<CallbackRequestResult | void> | void;

Both halves of the local path honour that. PassThroughStepImpl.handle reads the
result as modifiedReq?.response / modifiedReq?.method, and
PassThroughStep.serialize deliberately puts undefined on the wire for it:

const serializedResult = callbackResult
    ? withSerializedCallbackBuffers(callbackResult)
    : undefined;

The receiving half doesn't. PassThroughStepImpl.deserialize passed that value
straight into withDeserializedCallbackBuffers, which reads .body off it, and
typed the channel response as non-optional so the compiler couldn't see it
coming. Every inspection-only beforeRequest therefore failed its request when
the rule was running behind an admin server:

TypeError: Cannot read properties of undefined (reading 'body')

beforeResponse, a few lines further down, already guards exactly this with
if (callbackResult && typeof callbackResult !== 'string').

Reproduction

The remote equivalent of the existing test
"should be able to run a callback that checks the request's data":

await remoteServer.forGet(targetServer.url).thenPassThrough({
    beforeRequest: (req) => { seenMethod = req.method; }
});
await request.get(targetServer.url, { proxy: remoteServer.url });
// => StatusCodeError: 500 - "TypeError: Cannot read properties of undefined (reading 'body')"

Still reproduces on 4.6.3, the current release.

The change

beforeRequest now returns the callback result untouched when it is falsy, the
way beforeResponse does, and its channel.request type admits undefined,
matching what serialize() declares on the other end. No behaviour changes for
callbacks that do return a result.

Test plan

• Added the failing case to test/integration/remote-client.spec.ts, next to
  the other beforeRequest passthrough tests. Confirmed it fails with the
  error above before the fix and passes after.
• mocha 'test/**/*.spec.ts' (the test:node suite) on this branch: 830
  passing, 6 pending. Two full runs each produced one failure, but a
  different one each time (client-error-events header overflow, then
  https.spec.ts TLS-bypass timeout), and neither touches this code path -
  they look like pre-existing flakiness in this environment. A run on
  unmodified main was clean.
• Browser and perf suites not run locally.

[CLAassistant]

All committers have signed the CLA.

[philipp-winterle]

#214 fixes that flaky test