fix: apply ._strip_nulls to user ... in ._scale_xy_impl

scale_x/scale_y pass user ... through to the scale fun via
c(args, list(...)). If extra_args contains NULL values they can
trigger the same guide=none bug that .scale_colour_fun had.

Also replaces the inline NULL-strip on line 298 with the now-
centralized ._strip_nulls helper (code review finding #2).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: zorrooz

R/scale.R

@@ -295,8 +295,9 @@ NULL
   if (discrete && reverse && is.null(limits)) {
     args$limits <- rev
   }
-  args <- args[!vapply(args, is.null, logical(1))]
-  plot@gg <- plot@gg + do.call(scale_fun, c(args, list(...)))
+  args <- ._strip_nulls(args)
+  extra_args <- ._strip_nulls(list(...))
+  plot@gg <- plot@gg + do.call(scale_fun, c(args, extra_args))
   plot
 }