Index.sort_values puts missing values at the start with ascending=False
#### Code Sample, a copy-pastable example if possible

Index.sort_values places missing values at the start of the result with `ascending=False`

```python
In [4]: pd.Index([1, np.nan, 0]).sort_values(ascending=False)
Out[4]: Float64Index([nan, 1.0, 0.0], dtype='float64')

In [5]: pd.Series([1, np.nan, 0]).sort_values(ascending=False)
Out[5]:
0    1.0
2    0.0
1    NaN
dtype: float64
```
#### Problem description

This differs from Series.sort_values, which always leaves NA values at the end.

#### Expected Output

```python
Out[4]: Float64Index([1.0, 0.0, nan], dtype='float64')
```