ScrollBar: Hide rounded scrollbar if thumb is empty.

If a theme decides to make the minimized scrollbar be size
0 it should be completely hidden.

core/src/main/java/com/github/weisj/darklaf/ui/scrollpane/DarkRoundedScrollBarUI.java

@@ -73,10 +73,12 @@ public abstract class DarkRoundedScrollBarUI extends DarkScrollBarUI {
         }
         int arc = horizontal ? height : width;
         roundRect.setRoundRect(x, y, width, height, arc, arc);
+        if (!roundRect.isEmpty()) {
             g.setColor(getThumbColor());
             g.fill(roundRect);
             g.setColor(getThumbBorderColor());
             g.draw(roundRect);
+        }
         context.restore();
     }