Bugfix: remove unnecessary null initializations of shared_ptrs which lead to...

Bugfix: remove unnecessary null initializations of shared_ptrs which lead to compiler errors for gcc 4.3

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10285 2fb0f335-1f38-0410-981e-8018bf24f1b0