修复TagIpoutBUG

修复TextBox值为Null无法删除问题以及AllowDuplicates为false状态 VM中的Tags为null时的异常状态问题

demo/Ursa.Demo/ViewModels/TagInputDemoViewModel.cs

@@ -1,6 +1,13 @@
+using System.Collections.ObjectModel;
+
 namespace Ursa.Demo.ViewModels;
 
 public class TagInputDemoViewModel: ViewModelBase
 {
-    
+    private ObservableCollection<string> _Tags ;
+    public ObservableCollection<string> Tags
+    {
+        get { return _Tags; }
+        set { SetProperty(ref _Tags, value); }
+    }
 }