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:

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()