fix pred survival for single obs

[hfrick]

closes #255

library(censored)
#> Loading required package: parsnip
#> Loading required package: survival

decision_tree(engine = "rpart") %>%
  set_mode("censored regression") %>%
  fit(Surv(time, status) ~ ., data = lung) %>%
  predict(lung[2,], type = "survival", eval_time = c(100, 200))
#> # A tibble: 1 × 1
#>                .pred
#>   <list<tibble[,2]>>
#> 1            [2 × 2]

^{Created on 2023-04-11 with reprex v2.0.2}

[github-actions]

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