Ensure ComboBox smart hint behaves according to MD spec

According to the "Behavior" section of the "Menus" page in the link below, the hint should float even when there is no selection, but the popup is open.

https://m2.material.io/components/menus#exposed-dropdown-menu

Author: nicolaihenriksen

src/MaterialDesignThemes.Wpf/SmartHint.cs

@@ -258,7 +258,7 @@ private void RefreshState(bool useTransitions)
             string state = string.Empty;
 
             bool isEmpty = proxy.IsEmpty();
-            bool isFocused = proxy.IsFocused();
+            bool isFocused = HintHost?.IsKeyboardFocusWithin ?? proxy.IsFocused();
 
             if (UseFloating)
                 state = !isEmpty || isFocused ? HintFloatingPositionName : HintRestingPositionName;