fix: Null gravity override by Forge2dGame (#3092)

`Forge2dGame` overrides the world's gravity even when a gravity vector
is not provided to it. This causes the world to use the default gravity.

packages/flame_forge2d/lib/forge2d_game.dart

@@ -12,7 +12,7 @@ class Forge2DGame<T extends Forge2DWorld> extends FlameGame<T> {
     ContactListener? contactListener,
     double zoom = 10,
   }) : super(
-          world: ((world?..gravity = gravity) ??
+          world: ((world?..gravity = gravity ?? world.gravity) ??
               Forge2DWorld(
                 gravity: gravity,
                 contactListener: contactListener,

packages/flame_forge2d/test/forge2d_game_test.dart

@@ -78,6 +78,12 @@ void main() {
                   game.camera.viewfinder.zoom,
         );
       });
+
+      test("Game does not override World's gravity with null", () {
+        final game = Forge2DGame(world: Forge2DWorld(gravity: Vector2(10, 0)));
+        expect(game.world.gravity.x, 10);
+        expect(game.world.gravity.y, 0);
+      });
     },
   );
 }