title: UpdateSourceTrigger not working when TextBox becomes empty
body: <p>I have following XAML - </p>
  
  <pre><code>&lt;TextBox Text="{Binding Path=NumberOfItems, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" /&gt;
  &lt;Button Command="{Binding Path=StartCommand}"&gt;Start&lt;/Button&gt;
  </code></pre>
  
  <p>In ViewModel - </p>
  
  <pre><code>public string this[string columnName]
      {
          get
          {
              string result = null;
              switch (columnName)
              {
                   case "NumberOfItems":
                      if (this.NumberOfItems &lt;= 0)
                      {
                          result = "Items required";
                      }
                      break;
              }
  
              return result;
          }
      }
  </code></pre>
  
  <p>Whenever, TextBox changes values the trigger works accordingly. But in following cases, It is not working - </p>
  
  <ol>
  <li><p>When user presses delete button in keyboard after selecting whole text of textbox.</p></li>
  <li><p>When user deletes the last character present in TextBox.</p></li>
  </ol>
  
  <p>however ValidatesOnDataError is working.</p>
  
  <p>How can I make it work when TextBox empties?</p>
  
  <p>When user enters any invalid data, TextBox Style changes. I want this to be known in click of Start Button.</p>
  
  <p>How can I make aware the Start button that TextBox has invalid data?</p>