title: Laravel validation rule "URL" does not pass empty value
body: <p>I have input text field whith validation rule :</p>
  
  <pre><code>public function rules()
  {
      return [
          'field' =&gt; 'url',
      ];
  }
  </code></pre>
  
  <p>It`s not required (field can be empty) but validation return an error. </p>