Handle Case of No Sub-Orgunits and No Groups in Tenant

[rlxdev]

🗣 Description

The code changes made for this issue handle the case where the tenant has only the top orgunit - no sub-orgunits and no groups. It was discovered and reported by a user.

Closes #573

🧪 Testing

Tested internally via debugger, simulating that only the top-level orgunit existed without any sub-orgunits and groups.
The user who reported this issue also tested the proposed change and has confirmed that the changes resolve the issue.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• If applicable, All future TODOs are captured in issues, which are referenced in the PR description.
• The relevant issues PR resolves are linked preferably via closing keywords.
• All relevant type-of-change labels have been added.
• I have read and agree to the CONTRIBUTING.md document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge Checklist

• This PR has been smoke tested to ensure main is in a functional state when this PR is merged.
• Squash all commits into one PR level commit using the Squash and merge button.

✅ Post-merge Checklist

• Delete the branch to clean up.
• Close issues resolved by this PR if the closing keywords did not activate.

[buidav]

@rlxdev Can you designate reviewers for this PR?

[adhilto]

I tested on a tenant with no custom OUs or groups: worked as expected. I also tested on a tenant with a custom group but no custom OUs: also worked as expected. Great work.

[buidav]

No errors from running this branch against our normal test tenants either.
Note that the smoke test for just mac? is failing because of #604.
Smoke test failure will be addressed once that issue is resolved.