Adding `NULL` to a theme

[teunbrand]

This was identified in #6287 and affects 16 packages.

You can add NULL to a regular theme and it will be a no-op.
However, some packages don't also have the "gg" class for their themes, which causes problems when adding NULL.

library(ggplot2)

class(theme())
#> [1] "theme" "gg"
class(survminer::theme_survminer())
#> [1] "theme"

theme() + NULL
#>  Named list()
#>  - attr(*, "class")= chr [1:2] "theme" "gg"
#>  - attr(*, "complete")= logi FALSE
#>  - attr(*, "validate")= logi TRUE
survminer::theme_survminer() + NULL
#> Error in survminer::theme_survminer() + NULL: non-numeric argument to binary operator

^{Created on 2025-01-22 with reprex v2.1.1}

This can become a problem in the chunk of code below:

ggplot2/R/guides-.R

Lines 541 to 549 in 6a61201

	adjust <- NULL
	position <- groups$key$position[i]
	if (position == "inside") {
	adjust <- theme(
	legend.position.inside = groups$key$coord[[i]],
	legend.justification.inside = groups$key$justs[[i]]
	)
	}
	grobs[[i]] <- self$package_box(grobs[[i]], position, theme + adjust)