title: DevExpress SpinEdit shows 0 for null value
body: <p>I use MVVM in my project and in the ViewModel I use an int? value:</p>
  
  <pre><code>public int? Value1 { get; set;}
  </code></pre>
  
  <p>I have the following SpinEdit in my view:</p>
  
  <pre><code>&lt;dxe:SpinEdit Grid.Column="1" Grid.Row="1" MinValue="1000" MaxValue="2000"
  Name="Value1" IsFloatValue="False" NullText="Enter value" AllowNullInput="True" 
  NullValue="{x:Null}" MaskType="None" /&gt;
  </code></pre>
  
  <p>When I open the view, the NullText shows the correct value but when I enter some value into the SpinEdit and then <em>set the Value1 to null</em>, I get a 0 in my SpinEdit. As it has Min and Max value, it is invalid data.</p>
  
  <p><img src="https://i.stack.imgur.com/DKDxx.png" alt="enter image description here"></p>
  
  <p>What should I do to make the 0 value disappear <em>without the heavy event handler hackings</em>?</p>