Support an empty string passed as a status in `WP_Comment_Query`.

The changes in [30084] broke backward compatibility with interfaces that
manually passed an empty string for the value of 'status', such as on
wp-admin/edit-comments.php.

Fixes #29612.

git-svn-id: https://develop.svn.wordpress.org/trunk@30093 602fd350-edb4-49c9-b593-d223f7449a82