Historical Rules · 13 July 2013
Rule 509.4g
Official source text from the 13 July 2013 Rules release
An ability that reads "Whenever [this creature] attacks and isn't blocked, . . ." triggers if no creatures are declared as blockers for that creature. It will trigger even if the creature was never declared as an attacker (for example, if it entered the battlefield attacking). It won't trigger if the attacking creature is blocked and then all its blockers are removed from combat.