DataFrame populated with the letter "n" when no data passed and dtype is specified as str
This is discussed on Stack Overflow [here](http://stackoverflow.com/questions/28354207/why-is-an-empty-dataframe-of-dtype-str-filled-of-n/28355909).

When constructing a DataFrame with the `dtype` as `str` but not passing in any data (just an index), the resulting DataFrame is filled with the letter `n`:

``` python
In [61]: pd.DataFrame(index=range(2), columns=[0], dtype=str)
Out[61]: 
   0
0  n
1  n
```

This is contrary to the behaviour of Series when no data is passed in:

``` python
In [62]: pd.Series(index=range(2), dtype=str)
Out[62]: 
0    NaN
1    NaN
dtype: object
```

It would be logical for the DataFrame to be populated with `NaN` in this instance too.
