`pareto_khat(..., tail="both")` returns NA if just one tail is constant

[n-kall]

One problem is that when pareto_khat(..., tail="both") sees constant tail, max(left_k, right_k) gives NA. It would probably be better to use k <- max(left_k, right_k, na.rm=TRUE)

Originally posted by @avehtari in stan-dev/loo#250 (comment)