Someday, maybe even immediately after a high-profile stunt, Janet Jackson will release a lyrically substantial single that won't make it sound as if her larynx had been Botoxed. Alas, that time has not arrived with ''Just a Little While.'' The skittish, pared-down guitar opening is fresh and surprising, and the vibe is sexier than her Matrix Super Bowl Revolutions outfit. Ultimately, though, it's yet another lighter-than-air trifle in a career filled with too many of them; one longs for just a little of the lockstep tension of her early Jam and Lewis work. No wonder someone, whoever it was, felt the need to tweak the single with a little controversy.