Add more informative error if data has zero rows

[nrennie]

If data has been filtered and ended up with zero rows but the columns from and to still exist, this results in a non-informative error.

Example:

ggflowchart(tibble(from = c("A"), to = c("B")) |> filter(from != "A"))

returns:

Error in `dplyr::filter()`:
ℹ In argument: `.data$arrow_direction == "down"`.
Caused by error in `.data$arrow_direction`:
! Column `arrow_direction` not found in `.data`.
Run `rlang::last_trace()` to see where the error occurred.

Add a check somewhere.