title: FluentValidation how to check for Length if string is not null?
body: <p>I'm testing a <code>PUT</code> with two <code>string</code>:</p>
  
  <pre><code>company.CurrencyCode = request.CurrencyCode ?? company.CurrencyCode;
  company.CountryIso2 = request.Country ?? company.CountryIso2;
  </code></pre>
  
  <p>and I tried with a rule like:</p>
  
  <pre><code>public UpdateCompanyValidator()
  {
      RuleSet(ApplyTo.Put, () =&gt;
      {
          RuleFor(r =&gt; r.CountryIso2)
                .Length(2)
                .When(x =&gt; !x.Equals(null));
  
          RuleFor(r =&gt; r.CurrencyCode)
                .Length(3)
                .When(x =&gt; !x.Equals(null));
      });
  }
  </code></pre>
  
  <p>as I don't mind to get a <code>null</code> on those properties, but I would like to test the <code>Length</code> <strong>when</strong> the property is not a <code>null</code>.</p>
  
  <p>What is the best way to apply rules when a property is <code>nullable</code> and we just want to test if it's not null?</p>