Prevent default-constructed variants from holding a type

[adlarkin]

Signed-off-by: Ashton Larkin ashton@openrobotics.org

🦟 Bug fix

Summary

This is a follow-up to #370 (see that PR for a description of the bug fix). Since #370 breaks ABI and we cannot make this change in released versions without changing behavior and potentially breaking downstream users (relevant discussion: #370 (comment)), this fix is targeted for the next unreleased version, which is Fortress.

I also added some missing test coverage/updated documentation related to #358.

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

[codecov]

Codecov Report

Merging #371 (e2a3fee) into main (742bd27) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head e2a3fee differs from pull request most recent head 16014e7. Consider uploading reports for the commit 16014e7 to get more accurate results

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
+ Coverage   58.14%   58.16%   +0.01%     
==========================================
  Files         170      170              
  Lines       16788    16788              
==========================================
+ Hits         9762     9764       +2     
+ Misses       7026     7024       -2     

Impacted Files	Coverage Δ
include/ignition/rendering/Node.hh	100.00% <ø> (ø)
...e/ignition/rendering/base/BaseGaussianNoisePass.hh	96.66% <0.00%> (-3.34%)	⬇️
ogre2/src/Ogre2GpuRays.cc	95.61% <0.00%> (+0.57%)	⬆️

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[iche033]

looks good to me.