outcome_names doesn't work for recipes with NA roles

``` r
library(tidymodels)

rec <- recipe(mtcars) %>%
  update_role(disp) %>%
  update_role(mpg, new_role = "outcome")

outcome_names(rec)
#>  [1] "mpg" NA    NA    NA    NA    NA    NA    NA    NA    NA

rec <- recipe(mpg ~ disp, mtcars)

outcome_names(rec)
#> [1] "mpg"
```

<sup>Created on 2022-06-20 by the [reprex package](https://reprex.tidyverse.org) (v2.0.1)</sup>