Empty `rang` behaviors

[chainsawriot]

Example

graph <- readRDS("tests/testdata/rang_ok.RDS")
## similar to the case when all packages could't be resolved.
graph$ranglets <- list()

print looks okay.

convert_edgelist gives NULL (should it be an empty data.frame with from and to?)

export_rang gives an R file with an empty installation order. It runs but do nothing. (is it okay? Or at least some warnings?)

dockerize gives a directory. And building and launching it gives a vanilla R environment (it doesn't sound reasonable to me).

[schochastics]

see PR for convert_edgelist

I think export_rang should not produce a file at all given that there is nothing of use to be done with it?
Same for dockerize

So maybe both return nothing finishing with a warning that there is nothing to export/dockerize?