Previously, this was not possible due to an overly broad `empty()` check.
Passing `null`, `false`, or `''` to 'post_id', or omitting 'post_id'
altogether, will continue to return comments regardless of `comment_post_ID`,
as before. Passing `0` or `'0'` will limit results to comments with no
associated post.
Props danielbachhuber.
Fixes#35090.
git-svn-id: https://develop.svn.wordpress.org/trunk@36381 602fd350-edb4-49c9-b593-d223f7449a82