Revert "FrameSemantics: fix NullVertex warnings (#1458)"

[scpeters]

🦟 Bug fix

This reverts commit e40331e67301f88580dcdcdeba012832d3b239ab.

Summary

It appears that #606 and https://github.com/gazebosim/sdformat/pull/1458 caused test failures in gz-sim (see https://github.com/osrf/buildfarm-tools/issues/67#issuecomment-2232120738). I opened a test PR in gz-sim (https://github.com/gazebosim/gz-sim/pull/2482) that builds against gz-math8 and sdformat15 with those changes reverted, and the tests were fixed:

https://build.osrfoundation.org/job/gz_sim-ci-pr_any-homebrew-amd64/637/

I don't know why this broke tests, but I propose reverting these changes for now until we can figure it out.

Checklist

• [X] 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

Note to maintainers: Remember to use Rebase-and-Merge.