Fix crash when multicopter motor system is attached to an empty model

[iche033]

🦟 Bug fix

Fixes #2646

Summary

See instructions in #2646 for reproducing the crash.

Crash happens because the multicopter motor system is not able to find the parent link name component of a joint but still tries to access the component data. Fixed by adding a check for null component.

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

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.