`abstract` nuking columns inside ggplot objects

[brodieG]

p.base <- ggplot(data.frame(x=numeric(), y=numeric()), aes(x=x, y=y))
p.p <- p.base + geom_point()
str(abstract(p.p))
List of 9
 $ data       :'data.frame':    0 obs. of  0 variables
 $ layers     :List of 1