add padding cell in final gtable for NULL plot.margin (#6235)

Author: Yunuuuu

R/plot-build.R

@@ -286,7 +286,7 @@ ggplot_gtable.ggplot_built <- function(data) {
   plot_table <- table_add_tag(plot_table, plot$labels$tag, theme)
 
   # Margins
-  plot_margin <- calc_element("plot.margin", theme)
+  plot_margin <- calc_element("plot.margin", theme) %||% margin()
   plot_table  <- gtable_add_padding(plot_table, plot_margin)
 
   if (is.theme_element(theme$plot.background)) {