Drop zeroGrob() guides

[teunbrand]

This PR to the RC aims to fix a bug uncovered during reverse dependency checks.

Briefly, in some unnatural circumstances you can create a guide that survives all steps but decides to draw a zeroGrob().
Guides$package_box() cannot estimate the gtable size of these guides.

library(ggplot2)

ggplot(mpg[1:2, ], aes(displ, cty, colour = c(1, 4))) +
  geom_point() +
  scale_colour_continuous(
    limits = c(2.4, 2.6), breaks = c(1, 4),
    guide = "legend"
  )
#> Error in upgradeUnit.default(x): Not a unit object

^{Created on 2024-01-15 with reprex v2.0.2}

The solution in this PR is to drop empty guides at the Guides$draw() step. The reason we might not want to censor guides as soon as we detect the key is empty, is because some (extension) guides might have genuine reasons for not using a key.

[thomasp85]

LGTM