Comments: Restore inclusion of an empty comment type when building the `WHERE` clause in `WP_Comment_Query::get_comment_ids()`.

This ensures that `get_comments( array( 'type' => 'comment' ) )` still includes comments that have not yet migrated to the `comment` type.

Follow-up to [47597].

Props ocean90.
See #49236.

git-svn-id: https://develop.svn.wordpress.org/trunk@47625 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-comment-query.php

@@ -741,6 +741,7 @@ class WP_Comment_Query {
 
 					case 'comment':
 					case 'comments':
+						$comment_types[ $operator ][] = "''";
 						$comment_types[ $operator ][] = "'comment'";
 						break;