Check that stylesheets exist before adding them to settings array for watching

Drupal 6 automatically adds 'styles.css' to the page if the theme's info file doesn't declare any stylesheets. Subthemes inherit the stylesheets of their base themes. So if your base theme doesn't declare any stylesheets, Drupal wants to load styles.css from that theme.

Normally, Drupal checks that these files actually exist before adding them to the page, but CSS Watch does not. So it repeatedly requests a URL and the response is always 404.

I observed this with a sub-theme of Zen 6.x-2.x.