It's called bumping......it won't be blank, maybe a full stop or something similarMaybe its common, but see a lot of people pushing threads to be noticed for domain sales, by creating lots of blank messages. Is this aloud?
You can only do it a certain amount of times - try doing it more than allowed and you will get a auto alert saying your post has not been bumpedI guess the conclusion is: Bumping is fine, within reason