fix: keyboard navigation does not work when there is an item selected in FluentAutoComplete

[MarvinKlein1508]

🐛 Bug Report

Keyboard navigation is broken for the FluentAutoComplete component when Multiple is set to false and there is already an item being selected.

💻 Repro or Code Sample

https://www.fluentui-blazor.net/Autocomplete

1. Navigate to the multiple == false demo
2. Select an item (works with keyboard)
3. Click on the control again
4. Try to select an item with keyboard

🤔 Expected Behavior

I expect the keyboard to work the second time as well

😯 Current Behavior

You cannot select any item the second time with the keyboard

💁 Possible Solution

🔦 Context

This is necessary for the case of #4228 otherwise the FocusMethod will open a menu which cannot be navigated with keyboard.