fix for Page Title module and other modules that set head_title

I've started using the "Page Title" module (http://drupal.org/project/page_title), which naturally wants to set the `$vars['page_title']` variable, though the theme currently overrides that.

This pull request simply wraps the code that generates the page title in an `if(empty($vars['page_title']))` to determine if generating a page title is required.

Thanks!