avoid pujetid seg fault in extremeley rare case

The vertex association map producer didn't create an empty vector of candidates as an entry for a given vertex, if no tracks were ever associated (using dz) to that vertex.  This is technically possible but extremely rare, presumably for vertices with very bad fits, and it causes a seg fault in Pu Jet ID once in every O(10^4) events.  That caused all grid production to fail.  This PR fixes it.