MAINT: Cleanup ma.array.__str__ - #9768

>>> x = np.ma.array((1, '2006'), dtype=[('a', int), ('b', 'M8[Y]')])
>>> x
masked_array(data = (1, '2006'),
             mask = (False, False),
       fill_value = (999999, 'NaT'),
            dtype = [('a', '<i4'), ('b', '<M8[Y]')])

>>> x['a'] = np.ma.masked
>>> x
masked_array(data = (--, datetime.date(2006, 1, 1)),
             mask = ( True, False),
       fill_value = (999999, 'NaT'),
            dtype = [('a', '<i4'), ('b', '<M8[Y]')])

In [1]: x = np.ma.array((1, '2006'), dtype=[('a', int), ('b', 'M8[Y]')])

In [2]: x
Out[2]:
masked_array(data = (1, datetime.date(2006, 1, 1)),
             mask = (False, False),
       fill_value = (999999, 'NaT'),
            dtype = [('a', '<i4'), ('b', '<M8[Y]')])

In [3]: x['a'] = np.ma.masked

In [4]: x
Out[4]:
masked_array(data = (--, datetime.date(2006, 1, 1)),
             mask = ( True, False),
       fill_value = (999999, 'NaT'),
            dtype = [('a', '<i4'), ('b', '<M8[Y]')])