Handle filename better in ggsave()

[teunbrand]

This PR comes out of the discussion in #5114.

ggsave() gives some unhelpful error messages when the filename argument was inappropriate:

library(ggplot2)

p <- ggplot(mtcars, aes(mpg, disp)) + geom_point()
filenames <- c("plot1.png", "plot2.png")

# Casually plotting
ggsave(filenames, p)
#> Error in `ggsave()`:
#> ! `device` must be "NULL", a string or a function.

# Alright, let's try to fix the above error...
ggsave(filenames, p, "png")
#> Saving 7 x 5 in image
#> Error in if (!dir.exists(dir)) {: the condition has length > 1

# Might programmatically provide 0-length vector for some reason
ggsave(character(), p, "png")
#> Saving 7 x 5 in image
#> Error in if (!dir.exists(dir)) {: argument is of length zero

With this PR:

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

ggsave(filenames, p)
#> Warning: `filename` must have length 1, not length 2.
#> ! Only the first, 'plot1.png', will be used.
#> Saving 7 x 5 in image

ggsave(character(), p, "png")
#> Error in `ggsave()`:
#> ! `filename` cannot be empty.

^{Created on 2022-12-22 by the reprex package (v2.0.1)}

[yutannihilation]

Looks good, thanks!