I think the @null account is used by Steemit, Inc to receive the SBD when sell the Promoted Posts tab. I believe it is that some witness call "burning SBD". I saw that info in the Steem.center wiki page about Promoted Posts. Thanks and good luck again!
Thank you for your explanation, it seems you guys are right about it!
this is amazing I'm every day learning a new thing with steemit !!