test/run_nosetest.py: Return an empty string if PYTHONPATH is not defined.
test/run_nosetest.py: Return an empty string if PYTHONPATH is not defined.

DudLab/rank_filter,DudLab/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter,nanshe-org/rank_filter,jakirkham/rank_filter,jakirkham/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter