BUG: MultiIndex.unique incorrect when NA value is present
drop_duplicates looks right, while unique (and _get_unique_index) keep a duplicate entry containing an NA value.  Example based on test_intersection_with_missing_values_on_both_sides.  The analogous problem crops up for every entry in nulls_fixture

```
nulls_fixture = np.nan
mi1 = pd.MultiIndex.from_arrays([[3, nulls_fixture, 4, nulls_fixture], [1, 2, 4, 2]])

>>> mi1.unique()
MultiIndex([(3.0, 1),
            (nan, 2),
            (4.0, 4),
            (nan, 2)],
           )

>>> mi1.drop_duplicates()
MultiIndex([(3.0, 1),
            (nan, 2),
            (4.0, 4)],
           )
```