Chained transformations make empty iterators un-lazy

[rubensworks]

When transforming an empty lazy stream, the final stream becomes un-lazy.
This makes it so that the end-event can be emitted before any listeners can be attached.

Minimal reproducible example:

(async function() {
  // Create a lazy array it
  const arrayIt = new ArrayIterator([], { autoStart: false });

  // Transform it twice (both lazy)
  const transformedIt1 = arrayIt.transform({
    map: () => 1,
    autoStart: false,
  });
  const transformedIt2 = transformedIt1.transform({
    map: () => 1,
    autoStart: false,
  });

  console.log('WAITING...');
  setTimeout(() => {
    console.log('ATTACHING LISTENERS...');

    // The listeners below are never called
    transformedIt2.on('data', () => console.log('DATA'));
    transformedIt2.on('end', () => console.log('ENDED'));
  }, 1000);
})();