Apply on an empty dataframe exception
Apply to a DataFrame using lambda with a index/key (although that is in the DataFrame's index throws an error (a KeyError or IndexError respectively), perhaps it should exit more gracefully?

```
In [1]: x = pd.DataFrame(index=["a"])

In [2]: x
Out[2]: 
Empty DataFrame
Columns: array([], dtype=object)
Index: array([a], dtype=object)

In [3]: x.apply(lambda x: x["a"], axis=1)
```

``` ---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-80540f6c1d18> in <module>()
----> 1 x.apply(lambda x: x["a"], axis=1)

/Library/Python/2.7/site-packages/pandas-0.9.0-py2.7-macosx-10.7-intel.egg/pandas/core/frame.pyc in apply(self, func, axis, broadcast, raw, args, **kwds)
   3750             if not broadcast:
   3751                 if not all(self.shape):
-> 3752                     is_reduction = not isinstance(f(_EMPTY_SERIES),
   3753                                                   np.ndarray)
   3754                     if is_reduction:

<ipython-input-447-80540f6c1d18> in <lambda>(x)
----> 1 x.apply(lambda x: x["a"], axis=1)

/Library/Python/2.7/site-packages/pandas-0.9.0-py2.7-macosx-10.7-intel.egg/pandas/core/series.pyc in __getitem__(self, key)
    462     def __getitem__(self, key):
    463         try:
--> 464             return self.index.get_value(self, key)
    465         except InvalidIndexError:
    466             pass

/Library/Python/2.7/site-packages/pandas-0.9.0-py2.7-macosx-10.7-intel.egg/pandas/core/index.pyc in get_value(self, series, key)
    689                     raise InvalidIndexError(key)
    690                 else:
--> 691                     raise e1
    692             except Exception:  # pragma: no cover
    693                 raise e1

KeyError: 'a'
```

_I am trying to wrangle data from a huge number of Excel sheets (converting each sheet into a DataFrame but need to clean them before they are merged into something more sensible... I wanted to apply functions across these without worrying which are empty..._

(same error in 0.9.1.)
