[INTERNAL] fixed NPE in FileDecorator

It is possible that a SPath object references a resource that may no
longer available locally. Due the change of the file system we now face
null values when casting classes. This is the first patch to address
this issue.

Change-Id: I6cf2efec1999387ffa26c53a98e5ca34d80d6c30
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1502
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>