Use Apache's mod_alias module to perform a permanent redirect like:

RedirectPermanent / "http://world wide web.example.com/"

Put the redirect within the blank subdomain's virtualhost block or root level .htaccess file.

The trailing / on "http://world wide web.example.com/" is vital.

I'll add an alternative choice here that I have accustomed to permit other specific hosts too. For instance should you wanted a dev.example.com along with a blog.example.com site additionally for your world wide web.example.com site, put this inside your root level .htaccess file:

RewriteCond % .

RewriteCond % !^www.example.com

RewriteCond % !^dev.example.com

RewriteCond % !^blog.example.com

RewriteRule (.*) http://world wide web.example.com/$1 [R=301,L,QSA]