Check for empty lines after clipping

[paleolimbot]

Fixes #21. I think this is straightforward since you already check this in the same way before clipping.

I don't see a place where this was tested but would be happy to add a test if you can think of a good way to do it!

library(isoband)
library(grid)

m <- matrix(c(0, 0, 0, 0, 0, 0,
              0, 0, 0, 1, 1, 0,
              0, 0, 1, 1, 1, 0,
              0, 1, 1, 0, 0, 0,
              0, 0, 0, 1, 0, 0,
              0, 0, 0, 0, 0, 0), 6, 6, byrow = TRUE)

df_lines <- isolines((1:ncol(m))/(ncol(m)+1), (nrow(m):1)/(nrow(m)+1), m, 0.5)
grob_lines <- isolines_grob(
  df_lines,
  gpar(fontsize = 1000),
  label_placer = label_placer_manual("0.5", 0.5, 0.5, 0)
)

grid.newpage()
grid.draw(grob_lines)

^{Created on 2021-05-03 by the reprex package (v2.0.0)}

[clauswilke]

Yes, that's an obvious fix. Not sure how to test this, since this code is only run upon drawing. It's probably fine without.