runners: trace32: mypy fixes

Fixes for the following issues that a new version of mypy started
reporting recently:

runners/trace32.py:34: error: Incompatible default for argument
"startup_args" (default has type "None", argument has type
"List[str]") [assignment]

runners/trace32.py:137: error: Incompatible default for argument
"cfg" (default has type "None", argument has type "Path") [assignment]

runners/trace32.py:34: note: PEP 484 prohibits implicit Optional.
Accordingly, mypy has changed its default to no_implicit_optional=True

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>