title: Less if string is not empty conditional logic
body: <p>I'm using dotless to dynamically change the look of my site from an admin page.</p>
  
  <p>Essentially I use regular expressions to read the variables defined in the less and then give the user the option to change the value of the variables.</p>
  
  <p>I'm wanting to have the option to set a background image. Essentially I need a way to check if the string is empty if its not then add the background image mixin.</p>
  
  <pre><code>@BackgroundImage: '';
  
  .showBackground(@fileName) when (@fileName != '') {
      background-image: url('../Themes/images/backgrounds/@{fileName}');
  }
  
  body {
      .showBackground(@BackgroundImage)
  }
  </code></pre>
  
  <p>So the default is no background '' when the user sets a background the variable @BackgroundImage will be set to 'backgroundImage1.jpg'</p>
  
  <p>How can I get this empty string logic to work?</p>
  
  <p>P.S I've tried setting the variable to #000000 and using isstring() but it would appear to return true.</p>