I don't participate in the clubs and I don't actively curate those tags, so I'm not sure how the rules work for transfers to @null. Sorry. Maybe they just haven't thought to account for that, since post promotion has not been used much.

To promote a post, it is not necessary to also configure a null beneficiary setting. If you want to use the #burnsteem25 tag is when you would set a 25% beneficiary for @null.