Empty exports are not kept

[madkarmaa]

I'm creating a library that also has some submodules which extend the functionality of built-in types, so there's nothing to export
Example:

declare global {
    interface Array<T> {
        randomPick(): T;
    }
}

Array.prototype.randomPick = function <T>(): T {
    return this[Math.floor(Math.random() * this.length)];
};

export {};

When compiled using tsdown, the output of the .d.ts file looks like this

//#region src/array.d.ts
declare global {
  interface Array<T> {
    randomPick(): T;
  }
}

The empty export is missing, but it is required for the global declaration to take effect.