BUG: boxplot fails when one column is all NaNs

Fixes #8181. Currently the boxplot fails when trying to compute quantiles on an empty array, which numpy can't deal with. Works okay if we use an array with a single `np.nan` instead, the empty column is just left out of the plot as expected. 

I guess that might be a bit of a hack, relying on how numpy deals with a single `np.nan` in an array versus an empty array? If people think it's too risky to depend on that behaviour I'll try to rethink.

New output:

```python
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

df = pd.DataFrame(np.random.randn(100, 4))
df.loc[:, 0] = np.nan
df.plot(kind='box')
plt.show()
```

![missing_boxplot](https://cloud.githubusercontent.com/assets/1460294/4228586/39e763e4-395e-11e4-83d6-2f79fc12e4c3.png)