investigate `facet_var` specification in `g_boxplot.R`

https://github.com/insightsengineering/goshawk/blob/531047c9f2134e4c780a0c19912f68b24f950cf0/R/g_boxplot.R#L265

`facet_var` defaults to `NULL` for no faceting but can also be set to "None" for no faceting. This seems redundant. Investigate whether the "None" case can be discarded and simplify accordingly.

```
# Add facetting.
if (!is.null(facet_var)) {
if (facet_var != "None" & facet_var %in% names(data)) {
if (!is_finite(facet_ncol)) facet_ncol <- 0
if (facet_ncol >= 1) {
plot1 <- plot1 +
ggplot2::facet_wrap(stats::as.formula(paste0(" ~ ", facet_var)), ncol = round(facet_ncol))
} else {
plot1 <- plot1 +
ggplot2::facet_wrap(stats::as.formula(paste0(" ~ ", facet_var)))
}
}
}
```