DateTimePicker 'OK' button is enabled even when date is not selected

[nicolajb23]

Describe the bug
OK button on the date & time picker is enabled even when selected value is null

To Reproduce
Steps to reproduce the behavior:

1. Bind the value to a DateTime? with null as the default value
2. Open the dropdown
3. Observe the OK button is enabled (should be disabled)

Also would be nice to have a Today (select the date) and/or Now (select the time) buttons

Environment

• Blazority Version: 0.12.3
• .NET Core Version: .net 5
• Browser [e.g. chrome, safari]: N/A