[Combobox] Set value if selection is cleared

[joriverm]

Pull Request

📖 Description

this fixes #2302 , but is uncertain if it breaks anything again as the null coalesce might have been removed for a reason?

🎫 Issues

#2302

👩‍💻 Reviewer Notes

@vnbaaij , this is the fix i was talking about. Ive also added a unit test while i was at it

📑 Test Plan

Unit tests were added, but can also be tested by adding the code specified in the issue

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new compontent
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps

N/A

[joriverm]

@microsoft-github-policy-service agree

[vnbaaij]

I do not recall why the string.Empty was commented. I tried the PR change with the different combobox issues mentioned and they all work as they should still/now.