Update blog.php

Empty category blog did not show 'COM_CONTENT_NO_ARTICLES' message.
Ref. http://forum.joomla.org/viewtopic.php?f=624&t=723468
Fix in ~\components\com_content\views\category\tmpl\blog.php:
 5 lines added at line no. 46; checks 'show_no_articles' param as well.