title: Xaml PropertyChange event not fired when field is empty
body: <p>I have a textbox as a filter that takes a number value</p>
  
  <pre><code>&lt;TextBox Grid.Row="0" Grid.Column="5" Text="{Binding CourseNumber, Mode=TwoWay}"&gt;&lt;/TextBox&gt;
  </code></pre>
  
  <p>By Default this is set to 0 when the page loads. if i click search I bring back what is expected. If I then enter 204 into this textbox i get the appropriate items - only those items with a courseNumber of 204. </p>
  
  <p>Then if I delete anything in here - so its now empty and click search it is still holding onto this 204 value. the property change event is not being fired when I empty this field or when i empty the field and tab off it.</p>
  
  <pre><code>    private int _courseNumber;
      public int CourseNumber
      {
          get { return _courseNumber; }
          set
          {
              SetObject(ref _courseNumber, value, "CourseNumber");
              if (_courseNumber!= null)
              {
                  OnPropertyChange("CourseNumber")
              }
          }
      }
  </code></pre>
  
  <p>i can get this property change event to fire when inserting other numbers - just not when I removed all data from this field</p>
  
  <p><strong>Fix for us</strong>
  we wound up changing this property to a string instead of an int - then cast it in our search method - this worked perfectly.</p>