DataFrame.hist() does not get along with matplotlib.pyplot.tight_layout()
When `tight_layout()` is called after `DataFrame.hist()` it raises `AttributeError: 'NoneType' object has no attribute 'is_bbox` if any of subplots is empty (number of charts with histograms is smaller than number os subplots).

I.e. the following code:

```
pd.DataFrame({'a':np.random.randn(100),
              'b':np.random.randn(100),
              }).hist()
plt.tight_layout()
```

works fine, but this code:

```
pd.DataFrame({'a':np.random.randn(100),
              'b':np.random.randn(100),
              'c':np.random.randn(100),
              }).hist()
plt.tight_layout()
```

raises `AttributeError`.

My versions:
pandas: 0.15.2
numpy: 1.9.1
scipy: 0.14.1
IPython: 2.3.1
matplotlib: 1.4.2
