Fix a bug in sparse LAP tracker when linking frame to frame.

When no acceptable costs were found between two frames, the tracker
would fail with an error message similar to:
Tracking process failed:
At frame 7071 to 7072: [DefaultCostMatrixCreator] The row list is null
or empty.

This bug has been noticed and reported by several people, including
- Abbey Weith
- Rafael Buono
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Release v2.5.2.

- Fix a bug in the sparse LAP trackers.
- Fix the track spot quality feature analyzer.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Bump to next development cycle.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>