Fix grid output with unique groups in groupname_col and row_group_as_column = TRUE

[olivroy]

Split of.

Basically, just due to the fact that like base subsetting,

vctrs::vec_slice(c(1, 2), -integer(0))
#> numeric(0)
c(1, 2)[-integer(0)]
#> numeric(0)

which ends up creating an empty layout in this particular case. I have been bitten by this before ;)

# bug before this PR
df <- data.frame(x = 1:3, y = 4:6, row = c("A", "B", "C"))
gt(df, groupname_col = "row", row_group_as_column = T) |> plot()
#> Error in body[vertical] + n : 
 #> Non-numeric argument

This PR:

Split from #1801 as I am still trying to understand what is going on. What currently works, and what doesn't. I'd like to narrow down the issues and I thought this was a straightforward bug fix!

[rich-iannone]

LGTM!