BUG: barplot with NaNs

Closes https://github.com/pydata/pandas/issues/8175

This makes barplot more consistent with how NaN's are handled in AreaPlots and PiePlots.

We could try that... It would have to default to None, so that we could handle the different `kinds` differently.

But that seems like too much extra complexity when we already have the `fillna` method which has all the other abilities than just filling in a scalar.