Fix plugin conversion error message

[azeey]

🦟 Bug fix

Fixes #1555

Summary

This fixes the error

[Err] [Conversions.cc:1791] Tried to convert SDF [world] into [plugin]

The error comes from AddSystem passing an sdf::ElementPtr that corresponds to a <world> tag instead of a <plugin> tag when it's optional _sdf element is empty, which occurs mostly in tests when AddSystem is used to add Relay plugin which doesn't have an associated SDFormat file.

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)
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