FI-2790 Fix error caused by agent.who is nil

[yunwwang]

Summary

This is fix for GitHub issue https://github.com/onc-healthit/onc-certification-g10-test-kit/issues/514

Change Log

• Add & operator to agent.who
• Add unit tests to ensure when agent, agent.who, or agent.who.reference is nil, additional validation in ProvenanceValidator does not throw error

Testing Guidance

• all unit tests pass