Single-class box plot doesn't return any data

Not working snippet:

```py
import matplotlib.pyplot as plt
import numpy as np

data = np.random.randn(100)
bp = plt.boxplot(data, patch_artist=True)
plt.show()
```

Working snippet:

```py
import matplotlib.pyplot as plt
import numpy as np

data = np.random.randn(100)
data2 = np.random.randn(20)
bp = plt.boxplot([data,data2], patch_artist=True)
plt.show()
```