Retry fix for unsafe initialization in graph classes

[scpeters]

🦟 Bug fix

Follow-up to gazebosim/gz-math#612.

Summary

The is a second attempt at following up on the fix for potentially unsafe initialization issues in the graph::Edge and graph::Vertex classes after the first attempts in gazebosim/gz-math#606 and #1458 were reverted in gazebosim/gz-math#609 and #1459 due to test failures induced in gz-sim (documented in osrf/buildfarm-tools#67 (comment)). This pull request should be merged after gazebosim/gz-math#612, which uses a static local variable instead of dynamic memory allocation and doesn't cause gz-sim test failures in my testing.

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

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[scpeters]

I've merged gazebosim/gz-math#612 and rebuilt nightly debs, and CI is almost finished and all completed jobs have passed

[scpeters]

this will fix the clang warnings on main:

• https://build.osrfoundation.org/job/sdformat-ci-main-homebrew-amd64/194/