Reviewed by GREG KING Director: Gil Junger Stars: Martin Lawrence, Kevin Conway, Tom Wilkinson This unfunny and tedious variation on Mark Twain's classic tale A Connecticut Yankee At King Arthur's Court proves once and for all that the term "a Martin Lawrence comedy" is something of an oxymoron. (Okay, Big Momma's House, Bad Boys and Blue Streak were passably entertaining, but for the most part …