fix: Nullable value not working in `FluentTimePicker`

[hksalessio]

🐛 Bug Report

FluentTimePicker supports nullable values but when inserting and removing a value, the value stays at the last value (which is 00:00 because backspace sets it first back to 00:00 and then to --:--).
This is observable in the example on the demo page: https://www.fluentui-blazor.net/DateTime#defaulttimepicker

💻 Repro or Code Sample

2024-06-20-11-39-57.mp4

🤔 Expected Behavior

When using backspace to clear the value, it should still change to 00:00 first and when using backspace again it should change to null.

😯 Current Behavior

Using backspace twice leaves the value at 00:00.