physx: fix compile error

panda/src/physx/physxRevoluteJoint.cxx

@@ -242,7 +242,7 @@ set_limits(const PhysxJointLimitDesc &low, const PhysxJointLimitDesc &high) {
 PhysxMotorDesc PhysxRevoluteJoint::
 get_motor() const {
 
-  nassertr(_error_type == ET_ok, nullptr);
+  nassertr(_error_type == ET_ok, PhysxMotorDesc(0));
 
   PhysxMotorDesc value;
   _ptr->getMotor(value._desc);
@@ -255,7 +255,7 @@ get_motor() const {
 PhysxSpringDesc PhysxRevoluteJoint::
 get_spring() const {
 
-  nassertr(_error_type == ET_ok, nullptr);
+  nassertr(_error_type == ET_ok, PhysxSpringDesc(0));
 
   PhysxSpringDesc value;
   _ptr->getSpring(value._desc);