title: How to ignore validation annotation if field is blank?
body: <p>I have these classes:</p>
  
  <pre><code>Class Parent
  {
     [Required]
     String Name ;
  
     Child child ;
  }
  
  Class Child
  {
     [Required]
     Child FirstName ;
     [Required]
     Child LastName ;
  }
  </code></pre>
  
  <p>I have a form showing the Parent entity fields including the Childs's. With my configuration the child's FistName and LastName are required and if left blank causes validation to fail. </p>
  
  <p>What I need is have validation pass if both the child's FirstName and LastName are submitted blank. If either FirstName or LastName is submitted then the other is required and validation should fail. </p>
  
  <p>How can I achieve this?</p>