Allow renderer to be omitted in Webpack plugin

### Pre-flight checklist

- [X] I have read the [contribution documentation](https://github.com/electron-userland/electron-forge/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project uses.
- [X] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem description

I have a project in which I don't have a renderer.js, neither an index.html and so. But I would still like to bundle my source code.

Here is my project: https://github.com/felipecrs/clipboard-sync

### Proposed solution

To make `renderer` a optional option. Currently it's mandatory.

### Alternatives considered

Because of it, I'm not bundling anything and I'm just using `tsc` manually.

### Additional information

https://github.com/felipecrs/clipboard-sync/issues/14