Don't draw empty renderers

plot-lib/plot/private/no-gui/plot2d-utils.rkt

@@ -70,11 +70,10 @@
 
   (for ([rend  (in-list renderer-list)])
     (match-define (renderer2d rend-bounds-rect _bf _tf label-proc render-proc) rend)
-    ;; next step could be moved into (when render-proc, but this generates different step files)
-    (send area start-renderer (if rend-bounds-rect
-                                  (rect-inexact->exact rend-bounds-rect)
-                                  (unknown-rect 2)))
     (when render-proc
+      (send area start-renderer (if rend-bounds-rect
+                                    (rect-inexact->exact rend-bounds-rect)
+                                    (unknown-rect 2)))
       (render-proc area)))
 
   (send area end-renderers)

plot-lib/plot/private/no-gui/plot3d-utils.rkt

@@ -80,11 +80,10 @@
 
   (for ([rend  (in-list renderer-list)])
     (match-define (renderer3d rend-bounds-rect _bf _tf label-proc render-proc) rend)
-    ;; next step could be moved into (when render-proc, but this generates different step files)
-    (send area start-renderer (if rend-bounds-rect
-                                  (rect-inexact->exact rend-bounds-rect)
-                                  (unknown-rect 3)))
     (when render-proc
+      (send area start-renderer (if rend-bounds-rect
+                                    (rect-inexact->exact rend-bounds-rect)
+                                    (unknown-rect 3)))
       (render-proc area)))
 
   (send area end-renderers)