core: empty() for Rect/Size templates

Check for empty objects via .area() is not a good practice due overflows

modules/core/misc/java/src/java/core+Rect.java

@@ -65,6 +65,10 @@ public class Rect {
         return width * height;
     }
 
+    public boolean empty() {
+        return width <= 0 || height <= 0;
+    }
+
     public boolean contains(Point p) {
         return x <= p.x && p.x < x + width && y <= p.y && p.y < y + height;
     }

modules/core/misc/java/src/java/core+Rect2d.java

@@ -65,6 +65,10 @@ public class Rect2d {
         return width * height;
     }
 
+    public boolean empty() {
+        return width <= 0 || height <= 0;
+    }
+
     public boolean contains(Point p) {
         return x <= p.x && p.x < x + width && y <= p.y && p.y < y + height;
     }

modules/core/misc/java/src/java/core+Size.java

@@ -37,6 +37,10 @@ public class Size {
         return width * height;
     }
 
+    public boolean empty() {
+        return width <= 0 || height <= 0;
+    }
+
     public Size clone() {
         return new Size(width, height);
     }