Set SmartHint.IsContentNullOrEmpty on RefreshState

Author: huan.zhang

src/MaterialDesignThemes.Wpf/SmartHint.cs

@@ -265,6 +265,7 @@ private void RefreshState(bool useTransitions)
             else
                 state = !isEmpty ? HintFloatingPositionName : HintRestingPositionName;
 
+            IsContentNullOrEmpty = proxy?.IsEmpty() == true;
             IsHintInFloatingPosition = state == HintFloatingPositionName;
 
             VisualStateManager.GoToState(this, state, useTransitions);