When bass-baritone Thomas Quasthoff takes the stage at Lincoln Center tonight for a live national television broadcast with the New York Philharmonic, his appearance will be the first thing most viewers will notice, and yet, the announcer will offer not a single word of explanation. Quasthoff, a thalidomide baby, has vestigial arms and stunted legs, and there was a time in his career when he was willing, even eager, to discuss it. But these days, having long since established himself as a top vocalist, he prefers to let his music speak for itself, even if it sends a few thousand TV viewers scrambling for their search engines to dig up his story.