title: ASP.Net Web API: Allow blank strings in request property that has a MinLength attribute set
body: <p>I have a request field defined as follows:</p>
  <pre><code>[MinLength(9)] [MaxLength(18)] public string Id { get; set; }
  </code></pre>
  <p>I would like the user to be able to input a blank string for this field and not have it trigger the Minimum length attribute. Is there some other attribute that I can add for this or do I need to remove the MinLength attribute and do my own, more manual, validation?</p>