BUG: return Series as DataFrame.dtypes/ftypes for empty dataframes

`DataFrame.dtypes` and `DataFrame.ftypes` values were inconsistent for empty dataframes: 

```python
In [2]: pd.DataFrame().dtypes
Out[2]: 
Empty DataFrame
Columns: []
Index: []

[0 rows x 0 columns]

In [3]: pd.DataFrame().ftypes
Out[3]: 
Empty DataFrame
Columns: []
Index: []

[0 rows x 0 columns]

In [4]: pd.DataFrame(columns=list("abc")).ftypes
Out[4]: 
a   NaN
b   NaN
c   NaN
dtype: float64

In [5]: pd.DataFrame(columns=list("abc")).dtypes
Out[5]: 
a   NaN
b   NaN
c   NaN
dtype: float64

In [6]: pd.__version__
Out[6]: '0.13.0rc1-92-gf6fd509'

```