title: Only add unit on valid number
body: <p>I'm trying to do a mixin that have optional arguments. This is a simplified version:</p>
  
  <pre><code>@mixin marginCalculator($size, $size2:"") {
  
      margin: $size * 1px unquote($size2 * 1px);
  }
  </code></pre>
  
  <p>I'm just passing in numbers as arguments. But i only want the first one to be mandatory. No problem if it only would output the number but i need to add a unit to it. </p>
  
  <p>As can be seen in the snippet above i'm trying to multiplicate <code>1px</code> with <code>""</code> which ouputs an error message. As it should! But i was hoping for the empty quote to be returned. Is there another way to achieve this?</p>