Protection for position scale's call parameter

[teunbrand]

This PR to the RC aims to fix a bug uncovered in revdepchecks affecting ~44 packages.

One such example is this:
https://github.com/tidyverse/ggplot2/blob/rc/3.5.0/revdep/problems.md#newly-broken-29

Briefly, if a position scale is invoked in an non-standard way, where the first element of the call is not a name, it provoked an error.
To reproduce this error, the code below causes a function to be the scale's call[[1]]:

library(ggplot2) # RC branch

f <- function() {
  ggplot(mpg, aes(displ, hwy)) +
    geom_point() +
    scale_y_continuous()
}
do.call(f, list())
#> Error in as.character(call[[1]]): cannot coerce type 'closure' to vector of type 'character'

This is now wrapped in a try_fetch() block so that this does not cause a failure. If call[[1]] is not coercible to a character, it is presumed that the scale was not called as part of a scale-wrapper (like scale_y_log10()) and the call gets replaced with current_call().
This circumvents the error:

devtools::load_all("~/packages/ggplot2/") # This PR
#> ℹ Loading ggplot2
do.call(f, list())

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

[thomasp85]

Wouldn't it be better to check the type of call[[1]] rather than using a try_fetch?

[teunbrand]

I wasn't sure what types call[[1]] could assume. After testing a bit, I think most types, like name can be safely converted to character, but functions cannot.

[thomasp85]

LGTM