BUG: pandas.options.mode.use_inf_as_na disables NA checking for StringArray

1.0.3 or master:

```python
import pandas as pd

pd.options.mode.use_inf_as_na = True
arr = pd.array(["a", "b", None])
arr.isna()
# array([False, False, False])
```

Setting use_inf_as_na to True breaks isna for StringArray as shown above; it seems to work fine for other ExtensionArrays I checked (IntegerArray, BooleanArray).