FrameSemantics: fix NullVertex warnings

[scpeters]

🦟 Bug fix

Fixes deprecation warnings caused by https://github.com/gazebosim/gz-math/pull/606.

Summary

The gz::math::graph::Vertex::NullVertex static type has been deprecated in https://github.com/gazebosim/gz-math/pull/606 in favor of gz::math::graph::NullVertex(), so this fixes the deprecation warnings in FrameSemantics.cc by updating to the new API.

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)
• [X] codecheck passed (See contributing)
• [X] 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.

[scpeters]

@osrf-jenkins run tests please