[TimePicker] Add null check for empty string

[MarvinKlein1508]

Pull Request

📖 Description

This PR adds a null check for the FluentTimePicker which converts empty strings to DateTime?.

🎫 Issues

• fix: Nullable value not working in FluentTimePicker #2230

👩‍💻 Reviewer Notes

📑 Test Plan

I've added a new UNIT Test FluentTimePicker_EmptyStringToNull to verfiy the results.

✅ 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.

[MarvinKlein1508]

@dvoituron should be good now :)