Nobody’s suggesting Will Smith’s singing sucks, but a young chick CHOKED when he crooned her a tune — and that’s NO GAG! Shopping on Melrose Ave., the “Ali” star was passing the ’50s diner Johnny Rockets when a guy begged him to come inside and wish his girlfriend happy birthday. Nice-guy Will slipped up behind the babe and softly crooned in her ear. “Happy birthday to you . . . !” Stunned at suddenly being face-to-face with a superstar, the girl started choking on her burger! Folks slapped her on the back — but Will saved the day by performing the Heimlich maneuver. After downing a glass of water, Sweet Thing still couldn’t speak — but was smiling from ear to ear!