Issue an error during the WordPress install process if wp-config.php is...

Issue an error during the WordPress install process if wp-config.php is configured with an empty database table prefix, which is not supported.

props SergeyBiryukov.
fixes #19970.



git-svn-id: https://core.svn.wordpress.org/trunk@20699 1a063a9b-81f0-0310-95a4-ce76da25c4cd