fix(node-core): rejection warning in strict mode

[mohd-akram]

Only print it if the reason doesn't have a stack. This matches the behavior of Node.js.

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[isaacs]

I'm a bit confused. I don't think that this actually does match the behavior in Node...?

$ node --unhandled-rejections=strict -e 'Promise.reject("ok")'
node:internal/process/promises:281
    reason : new UnhandledPromiseRejection(reason);
             ^

UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "ok".
    at strictUnhandledRejectionsMode (node:internal/process/promises:281:14)
    at processPromiseRejections (node:internal/process/promises:413:17)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:32) {
  code: 'ERR_UNHANDLED_REJECTION'
}

Node.js v26.0.0

Maybe I'm missing the intention. What is the situation that this is supposed to address?

[mohd-akram]

@isaacs Okay, I don't think I've ever thrown a string as an error, but it seems the behavior is different in Node.js for Error objects vs strings:

$ node --unhandled-rejections=strict -e 'Promise.reject(new Error("ok"))'
[eval]:1
Promise.reject(new Error("ok"))
               ^

Error: ok
    at [eval]:1:16
    at runScriptInThisContext (node:internal/vm:219:10)
    at node:internal/process/execution:483:12
    at [eval]-wrapper:6:24
    at runScriptInContext (node:internal/process/execution:481:60)
    at evalFunction (node:internal/process/execution:315:30)
    at evalTypeScript (node:internal/process/execution:327:3)
    at node:internal/main/eval_string:71:3

Node.js v26.2.0

If it matters, I can adjust the PR to keep the current behavior if the object doesn't have a stack property, similar to the check that's already there.