JaqamanLinkingCostMatrixCreator: properly deal with no candidates.

There might be cases where none of the assignment candidates have a cost
below the cost threshold. In that case we pass en empty list to the
DefaultCostMatrix creator, which results in an error.
It is better to return a na assignment result.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
The Kalman tracker deals with the whole model.

It can now nucleate new KFs on the fly and kill some of them.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>