Fix crash when calling setPose service on static and nolink entity

[XINJIANGMO]

🦟 Bug fix

Fixes #2981

Summary

Gazebo allows loading static models without any links. However, if a /world/<world>/set_pose service call is made targeting such a model, the physics system crashes , because the underlying physics code assumes every model has at least one link.
After Fix : Before executing set_pose, the code now checks whether the target model contains at least one link.

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)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 68.95%. Comparing base (5aa528a) to head (8fc6f45).
Report is 5 commits behind head on gz-sim9.

Files with missing lines	Patch %	Lines
src/systems/physics/Physics.cc	33.33%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-sim9    #2988      +/-   ##
===========================================
- Coverage    68.97%   68.95%   -0.03%     
===========================================
  Files          345      345              
  Lines        33538    33572      +34     
===========================================
+ Hits         23134    23148      +14     
- Misses       10404    10424      +20     

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this stops set_pose from working with nested models

[iche033]

found that this changes in this PR stop set_pose from working with nested models, e.g.

    <!-- set_pose should work with this nested model -->
    <model name="box">
      <pose>0 0 2.5 0 0 0</pose>
      <static>true</static>
      <model name="inner_box">
        <pose>0 0 0.0 0 0 0</pose>
        <link name="link"/>
      </model>
    </model>

Did some debugging and noticed that the crash happens only in dartsim when a nested model tree has no links. A simple model (non-nested) with no links but cause the crash. I found the root cause and created a fix in gazebosim/gz-physics#765.

[XINJIANGMO]

Thank you for your correcting ! My PR really doesn't take nested models into account. Your fix is right !
In next few months, I'll continue to find and fix crashes for Gazebo, and let me know if you have any suggestions.

[iche033]

Thank you for your correcting ! My PR really doesn't take nested models into account. Your fix is right ! In next few months, I'll continue to find and fix crashes for Gazebo, and let me know if you have any suggestions.

great thanks for all your contributions!

[iche033]

oh this PR stops set_pos from working with a nested model, see #2988 (comment). Can we revert this?

[azeey]

Oh, sorry. Please go ahead and revert.

[iche033]

Oh, sorry. Please go ahead and revert.

created #3023 to revert this change