MoveIt visualization bug fixes

[JafarAbdi]

Description

This fixes two bugs

1- Using the name of the SceneNode was throwing an exception in the destructor see Mike's comment
2- Having an empty robot description will pass an empty string to rdf_loader which will cause it to throw an empty parameter name exception

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[mlautman]

I still think that the robot_description should be able to come from either a parameter, topic, or file like the RobotModelDisplay. Just using parameters doesn't make that much sense as it requires Rviz to be launched with special arguments