title: Making a Sass mixin with optional arguments
body: <p>I am writing a mixin like this:</p>
  
  <pre class="lang-css prettyprint-override"><code>@mixin box-shadow($top, $left, $blur, $color, $inset:"") {
      -webkit-box-shadow: $top $left $blur $color $inset;
      -moz-box-shadow: $top $left $blur $color $inset;
      box-shadow: $top $left $blur $color $inset;
  }
  </code></pre>
  
  <p>When called what I really want is that if no <code>$inset</code> value is passed, nothing is output, rather than it compiling to something like this:</p>
  
  <pre class="lang-css prettyprint-override"><code>-webkit-box-shadow: 2px 2px 5px #555555 "";
  -moz-box-shadow: 2px 2px 5px #555555 "";
  box-shadow: 2px 2px 5px #555555 "";
  </code></pre>
  
  <p>How do I rewrite the mixin so that if there is no value of <code>$inset</code> passed, nothing is output?</p>