gh-138813: Fix mutable default kwargs={} in multiprocessing BaseProcess and DummyProcess to use None (GH-138814)

* gh-138813: Default `BaseProcess` `kwargs` to `None` (#138814)

Set `BaseProcess.__init__(..., kwargs=None)` and initialize `kwargs` with
`dict(kwargs) if kwargs else {}`. This avoids a shared mutable default and
matches threading.Thread behavior.

Co-authored-by: Dmitrii Chuprov <cheese@altlinux.org>

* DummyProcess kwargs=None (which threading.Thread accepts properly)

Co-authored-by: Gregory P. Smith <greg@krypto.org>