fix(tables_figures): add missing !is.null()

accidentally removed when refactoring R code for name mapping, colour var needs to check if var is null, not just mapping

Author: amyheather

pages/experiments/tables_figures.qmd

@@ -726,7 +726,7 @@ plot_metrics <- function(
     # Helper to map a variable to display name if available in `name_mappings`.
     # Just uses variable name if no mapping is found.
     get_name <- function(var) {
-      if (!is.null(name_mappings[[var]])) {
+      if (!is.null(var) && !is.null(name_mappings[[var]])) {
         name_mappings[[var]]
       } else {
         var