What should `format_json_list` return for NA and NULL?

* no special treatment and simply let `format_json()` do its job.
* extra argument?

* `NA`: is unlikely to occur in a list. Maybe from `parse_json_vector()`?
* `NULL`: treated as the missing value in lists

```r
format_json_list(list(list(a = 1), list(a = 1, b = 2), NULL, NA))
#> {"a":[1]} {"a":[1],"b":[2]} {} [null]
```