`plot.initial_graph` breaks when `pairs` curvature is implicit and a paired edge curve is adjusted

```
wd_2005_graph <- graph_create(
    c(1 / 3, 1 / 3, 1 / 3),
    rbind(
      c(0, 1, 0),
      c(0, 0, 1),
      c(.5, .5, 0)
    )
  )

plot(wd_2005_graph, layout = "grid", nrow = 1, asp = .1, edge_curves = c("pairs" = 4, "H3|H2" = 6)) # OK
plot(wd_2005_graph, layout = "grid", nrow = 1, asp = .1, edge_curves = c("H3|H2" = 6)) # Not OK
```