fixed physicsmovementcomponent bug

source/core/src/main/com/csse3200/game/physics/components/PhysicsMovementComponent.java

@@ -11,7 +11,7 @@
 /** Movement controller for a physics-based entity. */
 public class PhysicsMovementComponent extends Component implements MovementController {
   private static final Logger logger = LoggerFactory.getLogger(PhysicsMovementComponent.class);
-  private Vector2 speed;
+  private Vector2 speed = Vector2.Zero;
   private PhysicsComponent physicsComponent;
   private Vector2 targetPosition;
   private boolean movementEnabled = true;