`stri_rand_strings()` should allow `length = integer()`

I've got some fuzzing code that generates random strings like this

```r
n_strings <- sample(0:10000, size = 1)
string_size <- sample(0:100, n_strings, TRUE)
stringi::stri_rand_strings(n_strings, string_size)
```

Notice how the length of `string_size` is equal to the number of strings, i.e. `n_strings`, so that it can be provided to `stri_rand_strings()`.

This fails when we get `n_strings = 0`:

```r
stringi::stri_rand_strings(0, integer())
```

I'd argue this should probably be allowed?