fix(anta): get_json_results returns null instead of "None"

[carl-baillargeon]

Description

get_json_results function in ResultManager was serializing None objects to 'None' instead of null.

It will now use the built-in model_dump from Pydantic for proper serialization.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have run pre-commit for code linting and typing (pre-commit run)
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (tox -e testenv)

[MaheshGSLAB]

LGTM