autoplot.fbl_ts legend problem when h=1

[robjhyndman]

Legend shading is incorrect when h=1. Perhaps related to #400

library(fable)

fit <- tsibble::tsibble(
    time = 1:20,
    value = rnorm(20),
    index = time
  ) |> 
  model(NAIVE(value)) 

fit |> forecast(h = 1) |> autoplot() 

fit |> forecast(h = 2) |> autoplot() 

^{Created on 2024-06-21 with reprex v2.1.0}