title: WPF TextBox IValueConverter ConvertBack not triggered when ValidationRules fail
body: <p>I have a TextBox which is linked to a double? in my backend viewModel. I need the ConvertBack to trigger when I empty the TextBox, but the validationRule "ValidateNotNullOrWhiteSpace" fail so there is no call to ConvertBack. My problem is that when I empty the textBox I want the viewModel to be null. Right now the viewModel keep the old value exemple 10.</p>
  
  <p>Is there a way to force the binding even when ValidationRules fail?</p>
  
  <pre><code>&lt;TextBox x:Name="txtSlabDepth"  Grid.Row="0" Grid.Column="1" Margin="0,3,0,0"
                               TextAlignment="Right" MaxLength="15" 
                               KeyDown="Textbox_KeyDown" TextChanged="Textbox_TextChanged" &gt;
                      &lt;Binding Path="SlabDepth"&gt;
                          &lt;Binding.Converter&gt;
                              &lt;converters:LengthInchTextboxConverter x:Name="LengthInchTextboxConverter_SlabDepth"  /&gt;
                          &lt;/Binding.Converter&gt;
                          &lt;Binding.ValidationRules&gt;
                              &lt;validations:ValidateLength x:Name="ValidateLength_SlabDepth"/&gt;
                              &lt;validations:ValidateNotNullOrWhiteSpace x:Name="ValidateNotNullOrWhiteSpace_SlabDepth"/&gt;
                          &lt;/Binding.ValidationRules&gt;
                      &lt;/Binding&gt;
                      &lt;TextBox.Style&gt;
                          &lt;Style TargetType="TextBox"&gt;
                              &lt;Style.Triggers&gt;
                                  &lt;DataTrigger Binding="{Binding Path=IsSlabDepthModified}" Value="True"&gt;
                                      &lt;Setter  Property="Background" Value="Yellow" /&gt;
                                  &lt;/DataTrigger&gt;
                              &lt;/Style.Triggers&gt;
                          &lt;/Style&gt;
                      &lt;/TextBox.Style&gt;
                  &lt;/TextBox&gt;
  </code></pre>
  
  <p>Thank you.</p>