Add dummy vertex to avoid accessing null ptr if no primary vertex found.

[ryonamin]

BEGINRELEASENOTES

• Add dummy vertex (0.,0.,0.) with error(0.,0., beam spot size in z) to avoid accessing null pointer if no primary vertex is found.
• This fixes #59. ENDRELEASENOTES

[ryonamin]

@tmadlener @gaede Thank you for pointing the problem out and suggesting solutions! My code worked unfortunately in my test and I missed the potential but crucial problem. Regarding the point Thomas suggested: "when the internal primaryVertices collection is filled from the LCIO contents", this idea sounds good to me but on the other hand, this fix leads us to reprocess the existing samples and this is a bit annoying, I guess. So I took Frank's solution and make a new pull request #61