Strengthened parameterized string tests.

The tests now make sure that the string representation of every class is not
none and also not ''.

The tests exclude a handmade list of classes for which '' is sometimes a valid
string representation:

    _allowed_to_be_empty_string = (
        indicatortools.Articulation,
        pitcharraytools.PitchArray,
        pitcharraytools.PitchArrayColumn,
        pitcharraytools.PitchArrayRow,
        pitchtools.Accidental,
        schemetools.SchemeColor,
        tonalanalysistools.ChordSuspension,
        )