Conan O’Brien shared a joint with weed-wielding comedian Seth Rogen on live television Tuesday. The smoky stunt was one of the late-night talk show host’s swan song shenanigans leading up to the finale of his TBS series, “Conan,” which will air its last episode Thursday after 11 seasons. (He’s been a late-night fixture for 28 years if you include his run on NBC.)“This is the kind of thing you do when you know it’s over for you,” O’Brien, 58, joked before lighting up the doobie that Rogen, 39,