PBS again must tiptoe through a political minefield as broadcasts the annual Mark Twain Prize for American Humor. This year was Julia Louis-Dreyfus’s turn to be feted at the Kennedy Center for the Performing Arts ceremony. She's best known these days for her work as vice president Selina Meyer on HBO's Veep, for which she already has been awarded six consecutive Emmys, as well as her nine years playing dance-challenged Elaine Benes on NBC's Seinfeld. But the Washington…