Import: empty tracks cannot be imported as TrackStatistics cannot be restored.

#316.

src/main/java/de/dennisguse/opentracks/io/file/importer/AbstractFileTrackImporter.java

@@ -257,6 +257,9 @@ abstract class AbstractFileTrackImporter extends DefaultHandler implements Track
         for (TrackPoint trackPoint : trackData.bufferedTrackPoints) {
             statistics.addTrackPoint(trackPoint, recordingDistanceInterval);
         }
+        if (!statistics.isTrackInitialized()) {
+            throw new ImportParserException("Track did not contain any locations.");
+        }
         trackData.track.setTrackStatistics(statistics.getTrackStatistics());
 
         Track track = contentProviderUtils.getTrack(trackData.track.getUuid());

src/main/java/de/dennisguse/opentracks/stats/TrackStatisticsUpdater.java

@@ -97,6 +97,10 @@ public class TrackStatisticsUpdater {
         return stats;
     }
 
+    public boolean isTrackInitialized() {
+        return trackInitialized;
+    }
+
     /**
      * Adds a trackPoint.
      *