a = np.array([['asdf', 'asdf'], ['asdf'], np.nan, ['asdf', 'asdf', 'asdf'],
    np.nan, ['asdf', 'asdf', 'asdf']], dtype=object)
a = to_none(a)
How can I set (dtype=object) numpy array values to Python lists, without numpy interpreting the lists as lists of values?