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.

OK. I've just realized it would probably be much more limited than `df.fillna()` since that can take `value=<scalar>` or methods. Also has limits, etc. I think that any argument in `df.plot(..., fillna=)` would only be able to handle scalars.