fix gtm writer issue with empty tracks. (#521)

fix gtm writer issue with empty tracks. (#521)

This format expects one track style entry for each tracklog entry
with the tracklog flag set.  If the tracklog flag is set it indicates the
start of a new track.  Each tracklog entry represents a point.
Thus, it is not possible to represent a track without any points in this
format.

This change prevents the writing of track style entries for tracks that
don't have any points.

Previously, this could cause counts from test-all in the category
"tests without error but with unexpected output" if the refernce file
had tracks with no points.