Bugfix for VariableTrackingPrecision, isEmpty was not implemented correctly. Instead of returning false in a Refineable Precision, the currently tracked variables are taken into consideration (such that if there is at least one tracked variable, isEmtpy() will return false, else true).

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14267 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c