fix(AutoCompleteBox):  PopulateDropDown handler is not unregister when MinimumPopulateDelay is set to Zero.
fix(AutoCompleteBox):  PopulateDropDown handler is not unregister when MinimumPopulateDelay is set to Zero.

AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia