Also accept empty array for ol.geom.MultiPolygon

The constructors of `MultiLineString` and `MultiPoint` both accept an empty array as default parameter, but the constructor of `MultiPolygon` throws an exception.
I think the fix should go in the `else` block:

```js
/**
 * @param {ol.geom.RawMultiPolygon} coordinates Coordinates.
 * @param {ol.geom.GeometryLayout=} opt_layout Layout.
 * @api
 */
ol.geom.MultiPolygon.prototype.setCoordinates =
    function(coordinates, opt_layout) {
  if (goog.isNull(coordinates)) {
    this.setFlatCoordinates(ol.geom.GeometryLayout.XY, null, this.endss_);
  } else {
    this.setLayout(opt_layout, coordinates, 3);
    if (goog.isNull(this.flatCoordinates)) {
      this.flatCoordinates = [];
    }
    var endss = ol.geom.flat.deflate.coordinatesss(
        this.flatCoordinates, 0, coordinates, this.stride, this.endss_);
    if (endss.length === 0) {
      this.flatCoordinates.length = 0;
    } else {
      var lastEnds = endss[endss.length - 1];
      this.flatCoordinates.length = lastEnds.length === 0 ?
          0 : lastEnds[lastEnds.length - 1];
    this.dispatchChangeEvent();
  }
};
```