fix: invalid Slack blocks with no PRs

[coltenkrauter]

Description

• Multi-Organization Support #27 fix the slack invalid_blocks error that happens when there are no open pull requests–The root issue was that the actions block was getting generated with an empty elements array when no GitHub Orgs were provided.
• add test to cover this scenario

Type of Change

• New feature
• Bug fix
• Chore
• Configuration
• Documentation
• Refactoring
• Hotfix
• Security
• GitFlow

Testing

• Executed locally
• Unit tests
• Integration tests
• Load tests
• PR Status checks
• Workflow execution (against non-default branch)
• Other (explain)
• Not worth it (explain)
• Not applicable

Checklist

[Not required to be completed, just check off what you've done.]

• Commits are reasonably sized and the commit message describes the changes well.
• I have made corresponding changes to the documentation.
• I have added unit tests for changes to business logic.
• I have emitted metrics so our team can monitor the health of this system related to these changes.
• I have added logs to ensure debugging is straight-forward when issues arise.
• I have considered failure modes (e.g. major dependency becomes unavailable, bad data passed in).
• I have added a comment to this PR to discuss any concerns/risks of these changes. This might include effects on performance, new dependencies, changes in behavior, or difficult things to test.
• I have considered whether some of these changes should go into a shared package to increase code reusability.