Dataframe ctor dies when passed a list containing empty dataframe
``` python
pd.DataFrame([DataFrame([])])

/home/user1/src/pandas/pandas/core/frame.pyc in __getattr__(self, name)
   1965             return self[name]
   1966         raise AttributeError("'%s' object has no attribute '%s'" %
-> 1967                              (type(self).__name__, name))
   1968 
   1969     def __setattr__(self, name, value):

AttributeError: 'DataFrame' object has no attribute 'dtype'
```
