[3.14] gh-138813: Fix mutable default kwargs={} in multiprocessing BaseProcess and...

[3.14] gh-138813: Fix mutable default kwargs={} in multiprocessing BaseProcess and DummyProcess to use None (GH-138814) (#139084)

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

* gh-138813: Default `BaseProcess` `kwargs` to `None` (GH-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.

* DummyProcess kwargs=None (which threading.Thread accepts properly)
(cherry picked from commit 5a15e7378996358848394930343e9633b6fec8a9)

Co-authored-by: Denis Sergeev <newjimbatler00@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>