plot.pie() autopct shows 0.00 for 0/None data
Not sure should I report this upstream to matplotlib to be honest but
The label correctly does not appear but the percent does,
```
percents = pd.Series(
    data = [
        0, # the same for None
        10
    ],
    index = [
        'nothing',
        'label'
    ],
    name = 'plot'
)

percents.plot.pie(autopct = '%.2f')
```
Output:
![madzijaplot](https://user-images.githubusercontent.com/4060824/79905951-4f6ed800-8417-11ea-968a-a983b7dca717.jpg)
