Don't write empty objects to GPX file

[jenetics]

The GPX definition allows to create empty objects like

final GPX gpx = GPX.builder()
    .metadata(md -> {})
    .addTrack(track -> {})
    .addTrack(track -> track
        .addSegment(segment -> {})
        .addSegment(segment -> segment
            .addPoint(wp -> wp.ele(12).lat(21).lon(23))))
    .build();

GPX.write(gpx, System.out, "    ");

This leads to empty elements in the serialized GPX file.

<?xml version="1.0" encoding="UTF-8"?>
<gpx version="1.1" creator="JPX - https://jenetics.github.io/jpx" xmlns="http://www.topografix.com/GPX/1/1">
    <metadata></metadata>
    <trk></trk>
    <trk>
        <trkseg></trkseg>
        <trkseg>
            <trkpt lat="21.0" lon="23.0">
                <ele>12.0</ele>
            </trkpt>
        </trkseg>
    </trk>
</gpx>

The files are valid, but empty elements doesn't add additional information. The library should omit empty objects when writing them into a file.

[jenetics]

Merged into r1.1.0 branch.