Stringify return object for getPropertyValue

[clairewu0218]

Pull Request Template

Description

Stringify return object for getPropertyValue() to align with the format of buildObjectModel for the new JSSDK.

Checklist

• [x] The code change is covered by unit tests. I have added tests that prove my fix is effective or that my feature works
• [x] I have performed end-to-end test locally.
• [x] New and existing unit tests pass locally with my changes
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I used clear names for everything
• [x] I have performed a self-review of my own code

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Microsoft.PowerApps.TestEngine	93%	90%	765	✔
Summary	93% (2020 / 2182)	90% (468 / 518)	765	✔

Minimum allowed line rate is 50%