KML writeFeatures error with some MultiGeometries

[mike-000]

Describe the bug
Another issue was found while testing a fix for #10614
MultiGeometries which have been read by ol/format/KML fail to write back if they contain MultiPolgons (or MultLineStrings or MultiPoints).

To Reproduce
Open the Drag and Drop example in codesandbox and add

  console.log(new KML().writeFeatures([event.features[0]], {
    dataProjection: 'EPSG:4326', featureProjection:'EPSG:3857'}));
  console.log(new KML().writeFeatures([event.features[1]], {
    dataProjection: 'EPSG:4326', featureProjection:'EPSG:3857'}));

to the addfeatures event handler then download https://github.com/openlayers/openlayers/blob/master/test/spec/ol/format/kml/states.kml and drag it to the example.

Alabama (a multigeometry consisting of a point and single polygon) is written back successfully.
Alaska (a multigeometry consisting of a point and multipolygon) fails.

Expected behavior
The writeMultiGeometry function needs to break MultiPolgons, MultLineStrings and MultiPoints within a non-homogenous geometry collection into their component parts as it does with homogenous MultiGeometry.