Convert empty .tck file to empty .rib
Convert empty .tck file to empty .rib

Fixes a bug where tckconvert saves an invalid .rib file.  If the
.tck file has less than 3 points, no curve nor points are written.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3