adapt to the NULL removal from rmw result validation string (#157)
adapt to the NULL removal from rmw result validation string (#157)

* adapt to the NULL removal from result valiation string

Signed-off-by: Ethan Gao <7ae19dff8e61f91e41232fe4de32cfe7f8857001@linux.intel.com>

* adapt to change in rcl and rmw validation result string

Signed-off-by: Ethan Gao <7ae19dff8e61f91e41232fe4de32cfe7f8857001@linux.intel.com>

* remove the check to return validation msg
because the unknown case doesn't return NULL now

Signed-off-by: Ethan Gao <7ae19dff8e61f91e41232fe4de32cfe7f8857001@linux.intel.com>

ros2/rclpy,ros2/rclpy