clean up setup_theme_function()

inst/scripts/generate_theme_functions.R

   body = "  eval(parse(text = call_style_xaringan()))"
 ) {
   if (file == "clip" && !requireNamespace("clipr", quietly = TRUE)) file <- ""
-  body <- c(
+  f_body <- c(
     "  # DO NOT EDIT - Generated from inst/scripts/generate_theme_functions.R",
     body
   )
   tv <- template
-  null_default <- purrr::map_lgl(tv$default, is.null)
-  tv[null_default, "default"] <- "{NULL}"
-  x <- c(
+  f_def <- c(
     "# Generated by inst/scripts/generate_theme_functions.R: do not edit by hand\n",
     as.character(
       glue::glue_data(
     "  outfile = \"xaringan-themer.css\"",
     ") {"
   )
-  if (!is.null(body)) x <- c(x, body, "}")
+  if (!is.null(f_body)) f_def <- c(f_def, f_body, "}")
   if (file == "clip") {
-    clipr::write_clip(x)
+    clipr::write_clip(f_def)
     message("Wrote ", f_name, " function signature to clipboard.")
   } else {
-    cat(reflow_roxygen(x), sep = "\n", file = file)
+    cat(reflow_roxygen(f_def), sep = "\n", file = file)
     message("Wrote ", f_name, " to ", file)
   }
   invisible()