Bugfix in TrkVertexFitters
The Trk::TrkVertexFitters codes will crash at AdaptiveVertexFitter.cxx#L937 and SequentialVertexFitter.cxx#L794 due to the fittedVertex is a null pointer.
The same bug found in Trk::TrkVertexBilloirTools have been fixed at !22479 (merged).
This merge request will do the same thing for Trk::TrkVertexFitters, it will return as soon as the fittedVertex is the null pointer.
Edited by Fudong He