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You don't have to specify positions all the time, but make sure to resize the variables, Have a look at following implementation,

  control_msgs::FollowJointTrajectoryGoal goal;
  goal.trajectory.joint_names.push_back("shoulder_pan_joint");
  goal.trajectory.joint_names.push_back("shoulder_lift_joint");
  goal.trajectory.joint_names.push_back("elbow_joint");
  goal.trajectory.joint_names.push_back("wrist_1_joint");
  goal.trajectory.joint_names.push_back("wrist_2_joint");
  goal.trajectory.joint_names.push_back("wrist_3_joint");
  goal.trajectory.points.resize(1); // important

  goal.trajectory.points[0].effort.resize(N_JOINT);
  goal.trajectory.points[0].positions.resize(N_JOINT);
  goal.trajectory.points[0].velocities.resize(N_JOINT);
  goal.trajectory.points[0].accelerations.resize(N_JOINT);

  for(size_t i = 0; i < N_JOINT; i++)
  {
    goal.trajectory.points[0].effort[i] = tau(i,0);
    goal.trajectory.points[0].positions[i] = 0;
    goal.trajectory.points[0].velocities[i] = 0;
    goal.trajectory.points[0].accelerations[i] = 0;
  }

  goal.trajectory.points[0].time_from_start = ros::Duration(1.0);
  goal.trajectory.header.frame_id = "base_Link";
  goal.trajectory.header.stamp = ros::Time::now()+ ros::Duration(0.001);
  traj_client_->sendGoalAndWait(goal, ros::Duration(0,0), ros::Duration(0,0));