Revert half of [19639]. If the blog_upload_space site_option is '0', restore...

Revert half of [19639]. If the blog_upload_space site_option is '0', restore 50 as a default. see #19538.

git-svn-id: https://core.svn.wordpress.org/trunk@19652 1a063a9b-81f0-0310-95a4-ce76da25c4cd