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

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.

New changeset 8ad052464a4e0aef9a11663b80f187087b773592 by Arturo Escaip in branch 'master':
bpo-40756: Default second argument of LoggerAdapter.__init__ to None (GH-20362)
https://github.com/python/cpython/commit/8ad052464a4e0aef9a11663b80f187087b773592

looks like this can be closed now?

Done.