增加了Null2VisibilityConverter转换器

value是Null的时候隐藏，不为Null时显示

src/Shared/HandyControl_Shared/HandyControl_Shared.projitems

@@ -209,6 +209,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\Color2HexStrConverter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\Int2StrConverter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\Long2FileSizeConverter.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\Null2VisibilityConverter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\Object2BooleanConverter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\String2VisibilityConverter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Tools\Converter\String2VisibilityReConverter.cs" />

src/Shared/HandyControl_Shared/Themes/Basic/Converters.xaml

@@ -16,5 +16,6 @@
     <converter:Int2StrConverter x:Key="Int2StrConverter"/>
     <converter:BorderClipConverter x:Key="BorderClipConverter"/>
     <converter:BorderCircularConverter x:Key="BorderCircularConverter"/>
+    <converter:Null2VisibilityConverter x:Key="Null2VisibilityConverter"/>
 
 </ResourceDictionary>

src/Shared/HandyControl_Shared/Tools/Converter/Null2VisibilityConverter.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace HandyControl.Tools.Converter
+{
+    public class Null2VisibilityConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return value == null ? Visibility.Collapsed : Visibility.Visible;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}