Comments: Restore inclusion of an empty comment type when building the `WHERE`...

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.
Built from https://develop.svn.wordpress.org/trunk@47625


git-svn-id: https://core.svn.wordpress.org/trunk@47400 1a063a9b-81f0-0310-95a4-ce76da25c4cd