bpo-40756: Default second argument of LoggerAdapter.__init__ to None (GH-20362)

The 'extra' argument is not always used by custom logger adapters. For
example:

```python
class IndentAdapter(logging.LoggerAdapter):
    def process(self, msg, kwargs):
        indent = kwargs.pop(indent, 1)
        return ' ' * indent + msg, kwargs
```

It is cleaner and friendlier to default the 'extra' argument to None
instead of either forcing the subclasses of LoggerAdapter to pass a None
value directly or to override the constructor.

This change is backward compatible because existing calls to
`LoggerAdapter.__init__` are already passing a value for the second
argument.

Automerge-Triggered-By: @vsajip