WKTWriter empty point/geometrycollection WKT serialization

[abrokenjester]

When putting an empty Point through WKTWriter.toString, it produces a string of the form POINT () or POINT(NaN, NaN) instead of POINT EMPTY. Similarly for an empty ShapeCollection, we get the serialization GEOMETRYCOLLECTION () instead of GEOMETRYCOLLECTION EMPTY.

[dsmiley]

Thanks for reporting. PRs welcome. Should be easy.