Add test case for zero retro value

[iche033]

Signed-off-by: Ian Chen [email protected]

🦟 Bug fix

Summary

A follow up to #522. Retro values for objects that do not have anything set should now return zero. Added a check and remove todo comment.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #527 (9ac202f) into ign-rendering6 (1505138) will not change coverage.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-rendering6     #527   +/-   ##
===============================================
  Coverage           54.40%   54.40%           
===============================================
  Files                 198      198           
  Lines               20084    20084           
===============================================
  Hits                10926    10926           
  Misses               9158     9158           

Impacted Files	Coverage Δ
ogre2/src/Ogre2GpuRays.cc	95.48% <ø> (ø)

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-01-citadel-edifice-fortress/1313/1