<TextBlock Text="{Binding Path=InstanceName}"></TextBlock>

<TextBlock
     Visibility="{Binding InstanceName, Converter={local:StringNullOrEmptyToVisibilityConverter}}"
     Text="{Binding InstanceName}"/>

public class StringNullOrEmptyToVisibilityConverter : System.Windows.Markup.MarkupExtension, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return string.IsNullOrEmpty(value as string)
            ? Visibility.Collapsed : Visibility.Visible;
    }
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return null;
    }
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }
}

<TextBlock Text="{Binding Path=InstanceName},FallbackValue={x:Null}"></TextBlock>

<TextBlock Text="{Binding Path=InstanceName}" Visibility="{p:PyBinding BooleanToVisibility(IsNotNull($[.InstanceName]))}"  ></TextBlock>

public class ViewModel
{
   public string InstanceName {...}
   public Visibility InstanceVisibility
   {
      get
      {
           return String.IsNullOrEmpty(InstanceName) ? Visibility.Collapsed : Visibility.Visible;
      }
}