Did you happen to watch any of the "Office" reruns last night on NBC? Over the credits, the show ran a clip of Nathan Alden Robinson from Newton playing the "Office" theme song on the the piano. Nathan, 15, died last month from flu-related complications. "The Office" was his favorite show. Here's the YouTube clip, which was filmed a few weeks before his death (and here is the Globe's sweet obituary). The song never seemed so sad.