Fix blank strip text with multilayered strips

[teunbrand]

This PR aims to fix #4050, wherein an error occurs in facet_grid() when the strip text is a blank element and there are two variables determining the strips. This also caused an inelegant warning in facet_wrap() that should now also be remedied.
Instead of returning an n by m matrix of zeroGrobs, the assemble_strips() internal function now returns an n-length list of zeroGrobs regardless of m. This then satisfies the FacetWrap/Grid$draw_panel() methods' assumption that the length of the strip-grobs is equal to the number of panels (for facet_wrap()) or columns/rows (for facet_grid()).

The following should now work without warnings or errors:

library(ggplot2)

df <- data.frame(
  x = 1, y = 1, a = "A", b = c("B", "C")
)

p <- ggplot(df, aes(x, y)) +
  geom_point() +
  theme(strip.text = element_blank())

# Was warning before, now doesn't warn
p + facet_wrap(~ a + b)

# Was error before, now runs fine
p + facet_grid(~ a + b)

[yutannihilation]

Looks good, thanks!

Could you add a news item? As at least two users faced this problem, I think this is worth a news.

[teunbrand]

Yes, sure thing. Thanks for approving!

[yutannihilation]

Thanks!