Hierarchical reindexing with NaN in index
related #6322
dupe of #7031 

I'm on 0.13.1, don't know if this is already fixed on master, but haven't found any issues about this. If I you have `NaN` value in MultiIndex, reindexing will fill the first value from reindexed series to all `NaN` indices. Perhaps an example will clarify it

Code:

``` python
import pandas as pd
import numpy as np

ix = pd.MultiIndex.from_tuples([('a',np.nan)], names=['col1', 'col2'])
pd.Series([1], index=[1]).reindex(ix, level=1)
```

Output:

```
col1  col2
a     NaN     1
```

Desired output:

```
col1  col2
a     NaN     NaN
```
