[DatePicker] Change OnSelectedDateAsync logic

[MarvinKlein1508]

Pull Request

📖 Description

This fixes the issue that the FluentDatePicker does not work, when the value of @bind is null. The issue comes from the fact that
(value ?? DateTime.MinValue).Date + Value?.TimeOfDay returns always null when Value is null

🎫 Issues

• fix: Datetime not selected after remove in Demo site #2197

📑 Test Plan

✅ 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.

[dvoituron]

Could you add a UnitTest to validate this particular case?

[MarvinKlein1508]

@dvoituron added UNIT-Test