[TST] 0-dimensional corner case

[arvoelke]

Would be good to get this unit test passing:

    sys = LinearSystem(1)
    ss_sys = LinearSystem(sys.ss)
    assert len(ss_sys) == 0

but the dimension can be 0 or 1 depending on whether before or after scipy==0.18.0 due to the way zero-dimensional state-space matrices are represented.