“Saturday Night Live” was more reflective than festive in the final episode of its 50th season. Scarlett Johansson, who set a record for a woman with her seventh appearance as host, used her monologue to lead most of the current cast of the NBC sketch institution in a song sung to the tune of Billy Joel's “Piano Man." The performance looked back on an eventful year that included an election, an epic anniversary special and a star-studded concert.