process: use defaults when spawning a child_process

[paul-marechal]

What it does

While the typings allow for undefined values, the behavior is actually
nasty: If you leave args undefined, then options are ignored. This
could lead to stdio pipes not being set correctly.

How to test

It happened to me when contributing a debug adapter using our internal contribution points:

@injectable()
export class SomeDebugAdapterContribution implements DebugAdapterContribution {
    provideDebugAdapterExecutable(): DebugAdapterExecutable {
        return {
            modulePath: 'somePath',
            // no `args` field, typings are ok with that.
        };
    }
}

In this case, the debug adapter was spawned, but nothing was happening. No errors too.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

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[paul-marechal]

Only this line might be necessary, but it doesn't cost much to provide sane defaults in the other places too.