Kiefer Sutherland is officially off the hook for that whole headbutting incident in NYC back in May ... because it amounted to a bunch of nothing. The Manhattan D.A. has just rejected the case against the "24" star and we're told it's all because officials determined it was a petty matter that did…

We've confirmed that Kiefer Sutherland will be charged with misdemeanor assault in connection with the headbutting incident earlier this week. Kiefer headbutted fashion designer Jack McCollough at an event after-party on Monday -- an attack Jack says was completely unprovoked. We're told Kiefer…