When you’re the son of a U.S. senator, all it takes is one open political seat to launch your next career. Robert “Rob” Menendez Jr.’s life so far has been expected, if not demanded: private high school, elite college, law school, and a career at a white shoe law firm. At 36 years old, he’s married with a young daughter and living in the Lafayette neighborhood of Jersey City. Flash forward a few years and he could be a partner or even running his own firm. But when Eighth District Rep. Albio Sires announced his retirement in December, Menendez Jr.’s career was set on a different path. Taking a…