Return an empty array from `get_approved_comments()` when `$post_id` is empty.

This behavior was broken when moving the internals to `WP_Comment_Query` in
[30098]. As a result, `get_approved_comments( 0 )` was fetching *all* approved
comments, causing performance issues.

Props dd32.
Fixes #30412.

git-svn-id: https://develop.svn.wordpress.org/trunk@30402 602fd350-edb4-49c9-b593-d223f7449a82