Ignore empty arguments in theme()

[teunbrand]

This PR to the RC aims to fix a bug uncovered in revdepchecks affecting ~32 packages.

One such example is this:
https://github.com/tidyverse/ggplot2/blob/rc/3.5.0/revdep/problems.md#newly-broken-126

Briefly, due to allowing splicing in theme(), we're no longer ignoring trailing arguments.
The fix is to explicitly ignore empty arguments using dots_list() instead of list2().

A reprex to reproduce the issue:

library(ggplot2) # RC branch

theme(axis.title = element_text(), )
#> Error in `list2()` at ggplot2/R/utilities.R:323:3:
#> ! Argument 1 can't be empty.

Fixed behaviour:

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

theme(axis.title = element_text(), )
#> List of 1
#>  $ axis.title:List of 11
#>   ..$ family       : NULL
#>   ..$ face         : NULL
#>   ..$ colour       : NULL
#>   ..$ size         : NULL
#>   ..$ hjust        : NULL
#>   ..$ vjust        : NULL
#>   ..$ angle        : NULL
#>   ..$ lineheight   : NULL
#>   ..$ margin       : NULL
#>   ..$ debug        : NULL
#>   ..$ inherit.blank: logi FALSE
#>   ..- attr(*, "class")= chr [1:2] "element_text" "element"
#>  - attr(*, "class")= chr [1:2] "theme" "gg"
#>  - attr(*, "complete")= logi FALSE
#>  - attr(*, "validate")= logi TRUE

^{Created on 2023-12-21 with reprex v2.0.2}

[thomasp85]

LGTM - surprised this causes any revdep failures though

[teunbrand]

You see the darndest things in revdep failures 🙃