What should `format_json_list` return for NA and NULL?

[mgirlich]

• 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

format_json_list(list(list(a = 1), list(a = 1, b = 2), NULL, NA))
#> {"a":[1]} {"a":[1],"b":[2]} {} [null]