Main page content block does not get placed properly by drupal_render_page.

http://api.drupal.org/api/drupal/includes--common.inc/function/drupal_re...

In cases where users disable the main page content block in their block configuration drupal has the weird behavior to print it anyhows (in the content region). Since this happens after hook_page_alter it places it inaprproriatly since we build the Omega page structure in hook_page_alter. Therefore we need to anticipate this behavior in cases where drupal_set_page_content was not invoked yet and do it in hook_page_alter before drupal_render_page can break it.