DAGs with no edges fail

[malcolmbarrett]

Per an accidental finding from CRAN tests, DAGs without edges fail. Kicking the can down the road for the CRAN version but will do this before the next release.

dagitty::dagitty("dag {
  x1
  x2
  x3
  x4
  x5
}") |> ggdag::tidy_dagitty()
#> Error in `dplyr::select()`:
#> ! Can't select columns that don't exist.
#> ✖ Column `x` doesn't exist.

^{Created on 2024-07-21 with reprex v2.1.0}