refactor: union output use str

[meskill]

Summary:
Briefly describe the changes made in this PR.

Issue Reference(s):
Fixes #... (Replace "..." with the issue number)

Build & Testing:

• [ ] I ran cargo test successfully.
• [ ] I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• [ ] I have added relevant unit & integration tests.
• [ ] I have updated the documentation accordingly.
• [ ] I have performed a self-review of my code.
• [ ] PR follows the naming convention of <type>(<optional scope>): <title>

[github-actions[bot]]

Action required: PR inactive for 2 days. Status update or closure in 5 days.

[tusharmath]

Too many open PRs at the moment. Lets come back to this one when we think we can improve performance in some way without introducing a leak.