Do not allow CounterexampleInfo.getTargetPathModel() to be null.
Analysis that cannot produce a model with information about variable
values can still use an empty model instance instead of null.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13435 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c