Ignore empty segments

[nathanvda]

I am using this library with pleasure in our software. Unfortunately a user of ours turned up with a gpx which contained empty segments at the end which caused an error.

E.g. something like

<gpx>
    <trk>
       [one or more correct segments]
        <trkseg>
        </trkseg>
        <trkseg>
        </trkseg>
    </trk>
</gpx>

When adding the segment to the track, the update_meta_data function barfed. My first solution was to only try to update the meta data if the segment actually contained points, but then the track also had empty segments, and not sure what the effects might be later on, so upon adding the segment I check if the segment actually contains points and only add in that case.

I added an example-file + test-case. This should have no effect imho for anybody else, it would just handle ill-formed gpx files better (well maybe ill-formed is worded too strong: but gpx files with empty segments).

[andrewhao]

Thanks for your contribution, @nathanvda!