title: jQuery-Validation-Engine - Custom funcCall validation causes error when value is empty
body: <p>I have a custom validation function to verify that two password fields match using jQuery-Validation-Engine:</p>
  
  <pre><code>&lt;input type="password" class="validate[required,funcCall[checkPassordMatch],maxSize[25]]"...
  </code></pre>
  
  <p>But if the field is not populated, and the form is submitted, I get the following exception:</p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'checkPassordMatch' of undefined 
  </code></pre>
  
  <p>Is there any way I can only run this validation only if the input is populated, or if "required" passes first?</p>
  
  <p>For reference, my function is as follows:</p>
  
  <pre><code>    // Validate the passwords match
  function checkPassordMatch(field, rules, i, options){
      console.log(field + ' : ' + rules + ' : ' + i  + ' : ' + options )
  
      if (field.val() != jQuery("#inputPassword").val()) {
          return options.allrules.passwordMatch.alertText;
      }
  }
  </code></pre>
  
  <p>Appreciate any thoughts.</p>