title: Fluent validation allow empty or value with structure
body: <p>I have a sort prop that can be empty or values like:</p>
  <pre><code>sort=&quot;&quot;
  sort=[&quot;name&quot;,&quot;desc&quot;]
  sort=[&quot;name&quot;,&quot;asc&quot;]
  
  
  RuleFor(r =&gt; r.Sort)
                  .NotEmpty()
                  .When(nr =&gt; nr.Contains(&quot;name&quot;))
                  .WithMessage(&quot;Invalid Sort&quot;);
  </code></pre>
  <p>How to allow empty, and non empty with contains a string?</p>