`geom_boxplot`: the plot is not drawn if there is a grouping and one of the categories contains only NaN

Example:

```python
data = {
'x': ['a', 'a', 'a', 'b', 'b', 'b', 'c'],
'y': [0, 1, 2, -1, 0, 1, None],
}
ggplot(data) + geom_boxplot(aes('x', 'y', fill='x'))
```

Instead of the plot, the message `"All data series in data frame must have equal size ..."` appears.

Without additional grouping there is no problem, see example:

```python
ggplot(data) + geom_boxplot(aes('x', 'y'))
```