assigning column values to empty dataframe with loc
I'm trying to add a column to an existing dataframe. The dataframe can be empty (0 rows) but I want the column to be added anyway. 

```
import pandas as pd
import numpy as np

df = pd.DataFrame(index=np.arange(4))
df.loc[:,'col'] = 42 # this works fine!

df = pd.DataFrame(index=np.arange(0)) # empty dataframe
df.loc[:,'col'] = 42 # ERROR: ValueError: cannot set a frame with no defined index and a scalar
df['col'] = 42 # this works fine but seems deprecated

pd.__version__ is '0.15.2'
```
