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.
Neither [MultiPoint](https://github.com/openlayers/ol3/blob/master/src/ol/geom/multipoint.js#L149-L162) nor [MultiLineString](https://github.com/openlayers/ol3/blob/master/src/ol/geom/multilinestring.js#L251-L265) seem to have a special check for `goog.array.isEmpty()`.