Allow child-themes to override comments.php

This fixes AHWEBDEV/Facebook-AWD-SEO-Comments/issues/2 by returning the input value to ``print_hidden_fbcomments()`` so that wordpress can continue to process the template file path.

Without this fix the wordpress function ``comments_template()`` receives a null/empty value from the ``comments_template`` filter and so uses the template in the parent theme.

See: http://core.trac.wordpress.org/browser/tags/3.5.1/wp-includes/comment-template.php#L910