Allow trailing commas in guides() and labs()

[yutannihilation]

Fix #4237

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

labs(x = "x", )
#> $x
#> [1] "x"
#> 
#> attr(,"class")
#> [1] "labels"
guides(x = "x", )
#> $x
#> [1] "x"
#> 
#> attr(,"class")
#> [1] "guides"

^{Created on 2020-10-15 by the reprex package (v0.3.0)}