BUG: Series.Count() miscounts NA values in ExtensionArrays
#### Code Sample, a copy-pastable example if possible

```python
In [9]: import numpy as np
   ...: import pandas as np
   ...: from pandas.tests.extension.decimal.array import to_decimal
   ...: ser = pd.Series(to_decimal([1,2,3]))
   ...: ser[0]=np.nan
   ...: ser.count()
3
```
