`col_` functions expand inputs of length 0

``` r
testthat::local_reproducible_output(crayon = TRUE)

length(cli::col_cyan(character()))
#> [1] 1
length(cli::col_cyan("a"))
#> [1] 1
length(cli::col_cyan(c("a", "a")))
#> [1] 2
```

<sup>Created on 2022-10-03 with [reprex v2.0.2](https://reprex.tidyverse.org)</sup>