By TIM MADIGAN FORT WORTH — On a recent Tuesday night at Bass Hall, the roll call of nations included Bulgaria, Israel, Italy, Korea, Russia, Australia, Germany, China and Japan. That night, when the semifinalists for the Cliburn International Piano Competition were announced, one nation was conspicuous by its absence: the United States.Again.Four years ago, Ning An was the lone American to advance to the semifinals but did not make the finals.