can't import web-worker inside a NodeJS worker thread with no workerData

Simply having an import to `web-worker` inside a NodeJS worker thread with no `workerData` will result in the following error:
```
node:internal/event_target:1101
process.nextTick(() => { throw err; });
^
TypeError [Error]: Cannot destructure property 'mod' of 'threads.workerData' as it is undefined.
at workerThread (file:///home/jonas/projects/temp/web-worker-in-worker-test/node_modules/web-worker/src/node/index.js:141:8)
at file:///home/jonas/projects/temp/web-worker-in-worker-test/node_modules/web-worker/src/node/index.js:75:94
at ModuleJob.run (node:internal/modules/esm/module_job:345:25)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:651:26)
at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:117:5)

Node.js v22.19.0
```

## Reproduction
### `main.js`

```js
import { Worker } from 'node:worker_threads';

const worker = new Worker('./worker.js', {});
setTimeout(() => worker.terminate(), 1000);
```

### `worker.js`
```js
import { parentPort } from 'node:worker_threads';
import Worker from 'web-worker';

parentPort.addEventListener('message', e => {
// omitted for brevity, as it is not relevant
});
```

## Notes

I saw that #40 tried to fix this issue already, but didn't fix it completely. The fix assumes `workerData` is set to a value. If it is `undefined`, we still run into the same issue as described in #40:

> [...] When using this library within a worker thread, the `workerThread()` code is executed, rather than the `mainThread()` code which creates the worker polyfill.
> [...]
> This change executes `workerThread()` only if the `mod` property is present from the `workerData`, indicating that it's being executed from a thread created within this library. If `mod` isn't present, this means that the code is executing within the context of some other worker thread that wasn't created from this library, and it needs to treat the current thread as the main thread by executing `mainThread()`.