Excpetion when the goal is rejected.

[teyssieuman]

https://github.com/BehaviorTree/BehaviorTree.ROS2/blob/cc31ea7b97947f1aac6e8c37df6cec379c84a7d9/behaviortree_ros2/include/behaviortree_ros2/bt_action_node.hpp#L549C3-L550C71

When the goal is rejected by the server, this goal_handle_ is nullptr. The function assync_get_result() then goes to an exception.

It seems that checking for goal_handle_==nullptr then returning correct this problem.