Improve interface TransferRelation.
Instead of one method with an optional (nullable) parameter,
define two different methods.
In most implementations, exactly one of the two cases
is relevant for the CPA, so this provides a clearer distinction
and better readable code with less uses of null.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13455 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c