ggraph still fails on some graphs without edges

[mschubert]

Related: #76, #82 (edge df also has 0 rows); #177 (same error message but different cause)

If I try the following code (updated CRAN and git HEAD for ggraph):

g = structure(list(1, TRUE, numeric(0), numeric(0), numeric(0), numeric(0),
   c(0, 0), c(0, 0), list(c(1, 0, 1), structure(list(), .Names = character(0)),
        list(name = "test"),
        list(dip_url = character(0), sources = list(), nrefs = integer(0)))),
   class = c("tbl_graph", "igraph"), active = "nodes")

ggraph(g)

I get the error:

Error: All columns in a tibble must be 1d or 2d objects:

• Column dip_url is NULL
• Column sources is NULL

Great package btw, it makes working with graphs so much easier! 👍