Prevent publishing empty messages in pose publisher (backport #3262)

[mergify]

🦟 Bug fix

Summary

Add a check to make sure we don't publish pose messages if it's empty, otherwise you get a bunch of empty msgs.

To test:

Add the pose publisher system to your favorite static model., e.g. edit shapes.df and add the following system to the ground_plane model:

      <plugin filename="libgz-sim-pose-publisher-system.so"
        name="gz::sim::systems::PosePublisher">
        <publish_link_pose>true</publish_link_pose>
        <publish_sensor_pose>false</publish_sensor_pose>
        <publish_collision_pose>false</publish_collision_pose>
        <publish_visual_pose>false</publish_visual_pose>
        <publish_nested_model_pose>true</publish_nested_model_pose>
        <publish_model_pose>true</publish_model_pose>
        <use_pose_vector_msg>true</use_pose_vector_msg>
        <static_publisher>true</static_publisher>
        <static_update_frequency>1</static_update_frequency>
      </plugin>

Run sim:

gz sim -v 4 -r shapes.sdf

Now echo the /model/ground_plane/pose topic to see that no messages should be published. Before this fix, there'll be a lot of empty messages (empty lines)

gz topic -e -t /model/ground_plane/pose

Echo the pose_static topic to make sure data are published:

/model/ground_plane/pose_static

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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This is an automatic backport of pull request #3262 done by [Mergify](https://mergify.com).