Having trouble publishing JointState msg from Arduino

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Hi everyone, I am currently creating my own robotic arm actuated with Dynamixel motors. The point of my code is to read the motor's position with an Arduino Mega and fulfill a sensor_msgs/JointState msg to be published over ROS on a /joint_state topic in order to get a feedback. Rosserial is usually working quite well, but due to some mistakes, I supposed in my arduino's code, the /joint_state topic is empty when I do a rostopic echo /joint_states (nothing is being published).

Here's my arduino code:

#include <DynamixelSerial.h> //rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0 _baud:=115200
#include <ros.h>
#include <sensor_msgs/JointState.h>

ros::NodeHandle  nh;

sensor_msgs::JointState robot_state;
ros::Publisher pub("joint_states", &robot_state);

int posJoint;

char robot_id = "arm";
char *joint_name[6] = {"joint_01", "joint_02", "joint_03", "joint_04", "joint_05", "joint_06"};
float pos[6];
float vel[6];
float eff[6];

void setup() {
  // put your setup code here, to run once:
  nh.getHardware()->setBaud(115200);
  nh.initNode();
  nh.advertise(pub);
  delay(1000);

  Dynamixel.setSerial(&Serial1); // &Serial - Arduino UNO/NANO/MICRO, &Serial1, &Serial2, &Serial3 - Arduino Mega
  Dynamixel.begin(1000000,2);  // Inicialize the servo at 1 Mbps and Pin Control 2
  delay(1000);
  Dynamixel.moveSpeed(1,512,512); //Home position
  Dynamixel.moveSpeed(2,512,512);
  Dynamixel.moveSpeed(3,512,512);
  Dynamixel.moveSpeed(4,512,512);
  Dynamixel.moveSpeed(5,512,512);
  Dynamixel.moveSpeed(6,512,512);

}

void loop() {
  // put your main code here, to run repeatedly:
  for(int id = 1; id <= 6; id++){ // Fulfill the arrays whith motor readings
    posJoint = posToRadians(Dynamixel.readPosition(id));
    pos[id] = posJoint;
    vel[id] = 0.5; // Value only for testing
    eff[id] = 0.1; // Value only for testing
  }

  // Fulfill the sensor_msg/JointState msg
  robot_state.name_length = 6;
  robot_state.velocity_length = 6;
  robot_state.position_length = 6;
  robot_state.effort_length = 6;

  robot_state.header.stamp = nh.now();
  robot_state.header.frame_id = robot_id;
  robot_state.name = joint_name;
  robot_state.position = pos;
  robot_state.velocity = vel;
  robot_state.effort = eff;

  pub.publish( &robot_state);
  nh.spinOnce();
  delay(10);

}

double posToRadians(int posInt) // Dynamixel position into radians
{
  float position_degrees = map(posInt, 0, 1023, -150, 150);
  float position_radians = (position_degrees * 3.14)/180;

  return position_radians;
}

Specs: - ROS melodic (up to date) - Ubuntu mate 18.04 - Arduino IDE 1.8.8

Feel free to ask me for more informations

Have a nice day