fix: include dummy-node in helper functions

[H-DNA]

Summary

• Cover cases in markInvalidNode & getMemberChain when the argument is a DummyNode.

Issue

(issue link here)

Lasting Changes (Technical)

(please list down: code changes/things that have wide-effect; new libraries/functions added that can be used by others; examples below)

• (Added class EmailValidator to validate email address' validity)
• (Added Tenant#is_trial? check)

Checklist

Please check directly on the box once each of these are done

• [ ] Documentation (if necessary)
• [X] Tests (integration test/unit test)
• [ ] Integration Tests Passed
• [ ] Code Review