[bfss] Make ``current_fs_path`` return None when an attribute has no value current_fs_path was returning the future fspath when no value was defined. * This does not make any sense * This tricks the storage into believing that updating the attribute content is not necessary. The file was written but the attributes was not updated. as a result the assignement was not made. (close #1725446)

# Run all scenarii found in windmill directory
from os.path import join, dirname
from cubicweb.devtools.cwwindmill import (CubicWebWindmillUseCase,
                                          unittest_main)

class CubicWebWindmillUseCase(CubicWebWindmillUseCase):
    #test_dir = join(dirname(__file__), "windmill/test_edit_relation.py")
    pass


if __name__ == '__main__':
    unittest_main()