Set defaults in text based keys

[teunbrand]

This PR fixes a bug introduced in #5562.

Briefly, I forgot to set proper defaults for some values that would make draw_key_text() and draw_key_label() incompatible with non-text geoms. In this PR, these defaults are set, so you can mix the text key with a point layer, for example.

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

ggplot(mpg, aes(displ, hwy, colour = factor(cyl))) +
  geom_point(key_glyph = "text")

ggplot(mpg, aes(displ, hwy, colour = factor(cyl))) +
  geom_point(key_glyph = "label")

^{Created on 2023-12-15 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

Thanks Thomas!