Fix WKT for empty geometry collections

[LachlanArthur]

Empty geometry collections should generate their WKT as GEOMETRYCOLLECTION EMPTY instead of GEOMETRYCOLLECTION()

$empty = new MatanYadaev\EloquentSpatial\Objects\GeometryCollection([], 4326);

$emptyWkt = $empty->toWkt();
// "GEOMETRYCOLLECTION()"

$parsed = MatanYadaev\EloquentSpatial\Objects\Geometry::fromWkt($emptyWkt, 4326);
// InvalidArgumentException: Invalid spatial value.

[MatanYadaev]

Thanks @LachlanArthur!