Blank text for guide_axis_theta()

[teunbrand]

This PR aims to fix #5530.

Briefly, it just early-exits when labels are blank and fills NULLs where values are expected.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  coord_radial()

p + theme(axis.text = element_blank())

p + theme_void()

^{Created on 2023-11-21 with reprex v2.0.2}

Should I add a test for this?

[teunbrand]

I realised downstream code does not expect a list of grobs, just a single one.

[thomasp85]

LGTM

[teunbrand]

Thanks for the review Thomas!