python: Fix optional parameters with default None
python: Fix optional parameters with default None

Change-Id: I40c80c8fdd1384b3405e6b1c0b0b9f310580568d
Signed-off-by: Kelly Nguyen <88e98a5bec5058930ec1fce4c3abb150a5efc2e9@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43605
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5