For AbstractBAMBasedRefiner.computePath() return null
instead of an empty dummy ARGPath when no path can be constructed.
The method computePath is allowed to return null anyway,
there dummy element was compared with ==
(so it can be replaced with null, too, because nothing relied on its
additional methods),
and empty paths are going to be forbidden.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13133 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c