color-swatch: render a background if the swatch doesn't have a color

We still want to call into the background rendering code, to draw the
default background.

gtk/gtkcolorswatch.c

@@ -145,6 +145,8 @@ swatch_draw (GtkWidget *widget,
 
       _gtk_theming_background_render (&background, cr);
     }
+  else
+    _gtk_theming_background_render (&background, cr);
 
   gtk_render_frame (context, cr,
                     0, 0, width, height);