Could map_dbl return empty vector for empty elements (revival)?

[DanChaltiel]

Hi,

This is a revival of #337:

Use .default = NA Originally posted by @hadley in https://github.com/tidyverse/purrr/issues/337#issuecomment-311067457

In fact, applying the closing comment does not solve the original question:

unlist(lapply(list(1), function(x) x[-1]))
#> numeric(0)
purrr::map_dbl(list(1), ~ .x[-1])
#> Error: Result 1 must be a single double, not a double vector of length 0
purrr::map_dbl(list(1), ~ .x[-1], .default=-99)
#> Error: Result 1 must be a single double, not a double vector of length 0

^{Created on 2021-10-23 by the reprex package (v2.0.1)}

It seems that .default is working when the result is NULL but not if the result is of length 0.

It would be very convenient if this were the case.