Preserve theme class

[teunbrand]

This PR aims to fix a revdep failure reported in #5822.

Subsetting a theme drops the class:

library(ggplot2)
class(theme())
#> [1] "theme" "gg"
class(theme()[])
#> [1] "list"

^{Created on 2024-04-03 with reprex v2.1.0}

We subsetted the theme in #5743 to drop unknown elements, but an extension (mine, guilty as charged) relied on classed behaviour of theme settings (invoking add_theme() via +). This PR replaced the subsetting operation with assigning NULL to unknown theme settings, preserving the class.

[thomasp85]

LGTM