Fix default bool in argparser
# What does this PR do?

As outlined in #12423, in a dataclass with no default for a bool parameter, the bool ended up defaulting to `True` when not passed, which is the opposite of the wanted behavior.
Fixes #12423
Do you want to pull in the test snippet from the issue to make sure it doesn't happen again? That test doesn't actually parse any arguments at the moment, so it relies on the argparser being configured right in the test, which is more error prone than just parsing simple things and checking them.