Fix partial match in deprecated guide arguments

[teunbrand]

This PR to the RC aims to fix a bug uncovered in revdepchecks affecting ~28 packages.

One such example is this:
https://github.com/tidyverse/ggplot2/blob/rc/3.5.0/revdep/problems.md#newly-broken-21

Briefly, title.position gets partially matched to legend.title instead of legend.title.position.
The fix is not to use a list() as theme placeholder but an empty theme() (where we disallow partial matching).

A reprex to reproduce the issue:

library(ggplot2) # RC branch

guide_colourbar(title.position = "top")
#> Error in `mapply()` at ggplot2/R/theme.R:536:3:
#> ! The `legend.title` theme element must be a <element_text> object.

In addition, I found that the reported call for mapply() was uninformative, so I wired some calls to validate_theme().
Now it reports the guide constructor.

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

# Correctly has `legend.title.position` now
x <- guide_colourbar(title.position = "top")
x$params$theme
#> List of 1
#>  $ legend.title.position: chr "top"
#>  - attr(*, "class")= chr [1:2] "theme" "gg"
#>  - attr(*, "complete")= logi FALSE
#>  - attr(*, "validate")= logi TRUE

# New call
guide_colourbar(title.theme = element_rect())
#> Error in `guide_colourbar()`:
#> ! The `legend.title` theme element must be a <element_text> object.

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

[thomasp85]

LGTM