Add ExtesnsionDtype.empty classmethod
This would let us provide a default `construct_from_string` method.

> It seems we could have a default implementation for ExtensionDtype.construct_from_string ? (I now just copy pasted from the decimal example, and I think json example also has the same basic one)

The default would have to rely on `ExtensionDtype()` being constructable with no arguments, or an `ExtensionDtype.empty` method. For many that'll be

```python
@classmethod
def empty(cls):
    return cls()
```

which is easier than implementing `construct_from_string`. Maybe call it `from_empty`.