Add `allow_empty_list` argument to `spec_*()`

[mgirlich]

So that the following can be tibblified

jsonlite::fromJSON('[{"a": [1, 2]}, {"a": []}]', simplifyDataFrame = FALSE)
#> [[1]]
#> [[1]]$a
#> [1] 1 2
#> 
#> 
#> [[2]]
#> [[2]]$a
#> list()

^{Created on 2022-06-23 by the reprex package (v2.0.1)}

This probably only affects tib_vector(input_form = "vector") so think about the name.
Check the support in the guessing API.