prevent absolutizing a relative config path when it is empty
prevent absolutizing a relative config path when it is empty

The result was that the config path was erroneously being set to the root /
This specifically happened when downloading WordPress via `wp core download`

See comment by @scribu: https://github.com/wp-cli/wp-cli/pull/362#issuecomment-15480546

wp-cli/wp-cli,wp-cli/wp-cli