fix sosy-lab/benchexec#661: do not return None but empty list
fix sosy-lab/benchexec#661: do not return None but empty list

sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,dbeyer/benchexec,sosy-lab/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec