appending empty dataframe with columns fails
I am trying to append an empty dataframe to another empty one. This fails if at least one of them has a column defined. Below is the code snippet to generate the error.

```
import numpy as np
import pandas as pd
pd.__version__
#'0.10.1'
np.__version__
#'1.7.0'

a = pd.DataFrame()
b = pd.DataFrame()
a.append(b) # works

a = pd.DataFrame(columns=['asdf'])
b = pd.DataFrame(columns=['asdf'])
a.append(b) # fails

a = pd.DataFrame(columns=['asdf'])
b = pd.DataFrame()
a.append(b) # fails

a = pd.DataFrame()
b = pd.DataFrame(columns=['asdf'])
a.append(b) # fails
```

Exception is: ValueError: need at least one array to concatenate
