feat: Add nullable = false to variables with defaults in submodules

[abohne]

Description

This PR sets nullable = false on variables in the policy and rule-group submodules. This causes terraform to use the default variables instead of null when using the submodules outside of this module.

Motivation and Context

If you are trying to use the policy and/or rule-group submodules outside of the main module, you cannot utilize the defaults in the submodules' variables by using null as the value in a variable.

Breaking Changes

This should not break anything

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects
  I applied the complete example and also validated that the submodules behave as expected when null values are passed into them.
• I have executed pre-commit run -a on my pull request

[abohne]

@bryantbiggs Not sure if you're the correct person to tag, but would be nice to get this reviewed/merged

[bryantbiggs]

its not going to be something added fairly quickly - theres quite a bit more work involved before this would be added

[abohne]

What additional work is required?

[github-actions]

This PR has been automatically marked as stale because it has been open 30 days
with no activity. Remove stale label or comment or this PR will be closed in 10 days

[github-actions]

This PR was automatically closed because of stale in 10 days

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.