Render comments and form to see if headings are needed

Currently, the Zen theme displays the "Comments" heading even if there aren't any comments. Core checks to see if ($content['comments']) before printing the heading, but that is flawed since that array may be non-empty yet not render to anything.

Same is true for the comment form.

So we need to pre-render both the $content['comments'] and the $content['comment_form']. :-p